As seen in the comments, it looks like your camera us unaware of the flash, at least in some operating modes. Research in your user's manuals, there may be an answer. If not, Maybe your camera/flashgun combo is not able to use TTL (trhough the lens) flash regulation.
Another possibility is that some of the hotshoe contacts is not working (i.e. dirty) and that is preventing the functionality to operate properly. (Provided that your flash is actually 100% compatible with your camera).
LEt me briefly introduce a couple of useful concepts: Types of light regarding its time duration.
Continuous Light: Is the one coming from a rather constant source, like the sun, an incandescent bulb, a stable, flame, etc. As you can see, this kind of light can be considered constant, as it's intensity will not vary a lot through the time that takes a "normal exposure".
Pulsating Light: For photographic purposes it is light coming from flashes and strobes. These are pretty obvious to explain, but consider that the light pulse duration is very close to the exposure time for a typical shot,
The previous is very important because of the way difrent types of light interact with the light regulation methods your camera has:
Aperture: It regulates the amount of light that enters throug the "iris" at any given time. For example, an aperture one stop "more closed" lets in "half the light", no matter if the time considered is a second, a minute, a day... as long as both measurements are taken with the same time. Remember that Aperture also controls the depth of field.
Shutter Speed: It controls the amount of light by means of controlling for how long this light is let in. It means that a shutter speed "one stop faster" opens the shutter for half the time, thus letting in only half the light (assuming a perfectly fixed continuous source of light)
Sensitivity: This is the ISO setting. This does not affect the amount of light but how the recording medium reacts to a given amount of light, no matter if it's analog (film) or digital (sensor).
When you work in a rather luminous location but are also using flash, you are combining two types of light, continuous and pulsating, thus, you can control how much of every type of light reaches your sensor of film by choosing the right method: Aperture or Speed.
One of my photography teachers used to say: "You control continouos light with shutter, pulsating light with aperture".
Shutter speed affects "only" continuous light. Pulsating light is theoretically unnafected by shutter speed because the pulse has a fixed duration. This duration is extremely short, so adding more time to the exposure only lets in more continuous light, if there is any. However, there are practical limitations due to mechanical and electronic delays and response times. Normally, as long as you do not exceed your system's synch speed you'll be fine. (This speed usually hits between 1/125 - 1/180, but there are a lot of exceptions)
Aperture affects both, continuous or pulsating light, and also controls depth of field. Sensitivity also affects both, but has a side effect in contrast quality and grainyness of the final image.
When you shoot a scene, you have to define what's going to be your main light, the dominating light, and set your camera accordingly. Usually it is "easier" in such difficult conditions to work in Manual mode, since you can control all three parameters: Shutter speed, aperture and sensitivity.
Aperture mode will most of the time give you a shutter speed that will catch enough ambient light to get a "properly exposed" picture. If That ambient light plus your flash's light is giving you a picture way over exposed, it means your scene is likely iluminated enough without the flash. This is because the flash's pulse duration is likely fixed and very high, so, decreasing shutter speed (keeping it open more time) just lets more ambient (continuous) light in.
If you choose a more closed aperture (higher F number) the camera will let in less light, both ambient and flash, but also will increase exposure time, so after the flas pulse finishes, it will continue gathering ambiend light. If the subject or the camera moves it will result in ghosting. If you choose a more open aperture (low F number) the camera will le in more light and reduce exposure time, likely resulting in over exposure)
If you choose Shutter priority mode (S or Tv) and go to a high speed seting, the camera will open up the iris (lower the F number), as you go increasing the speed, the less ambient light will get in your picture, you will notice how the flash now dominates the scene, but if you still get too much ambient light, you will eventually hit your system's sync speed limit (the camera wont let you set a faster shutter)
Try going to manual mode, use values aproximately like these: Iso 100, F 4.0, Speed 1/80-1/125, and shoot without flash. If your scene results close to well exposed, then, that's the problem, there is too much ambient light. These values I give are my start point when I try to shoot indoors at night (under a typical home ilumination) with flash at aproximately 3 meters/10 feet.
If with these settings you still want to use the flash then:
- Increase shutter speed so you get less ambient light in the picture, your test shots without flash will be clearly under exposed, but remember there is a limit given by the synch speed.
- Use a small aperture to reduce both lights (ambient and flash) until your'e satisfyed with the result.
Now, depending on your intended result, you may also try:
- Change shooting time, so the sun is in a different angle, giving you
less intense ambient light in your location.
- Change location for a less iluminated one.
- Reorient the beam of your flash so it bounces the ceiling or a wall instead of hitting directly on your subject.
- Use a difuser in front of your flash so the light is softer, broader and less intense.
- Place the flash further away from the subject (If it's on your camera's hotshoe then step back and zoom in, it also helps reducing perspective distortion and blurring the background)
EDIT: I did purposely let out that you can also regulate your flash's power. Usually your camera will let you set 1/1, 1/2, 1/4, 1/8, 1/16, 1/64 of its power (some cameras may not allow all the setings or may have more setiings), either for the integrated flash or the external one. If the flash is external, you can also set it in manual mode and set the power level there.