You cannot trigger it by flash. The SB-600 does not have an optical slave mode. You can buy an inexpensive optical slave trigger though and trigger it with the built-in flash on your camera.
You may be able to mount it in the hotshoe of a non-Nikon body, but it may not fire. In fact, it's possible it could damage the camera. Nikon flashes use 12V trigger ...
Another option to consider is getting one pair of radio triggers and then using an optical device like the Wein Peanut to make the second flash into a slave, if it doesn't already have that capability. I've used this kind of setup to fire extra flashes and stretch my existing wireless trigger capability.
Note that if you go with the wired trigger bridged ...
If you have the PC-Sync cable connectors (either directly to the camera and flashes or indirectly through a hot-shoe converter), and have minimal soldering skills, you can cut a PC-Sync cable and solder standard audio 3.5mm phone jacks so you can disconnect it and do all sorts of nice things. For example, use a standard phones extension cables to extend the ...
Surprisingly, but after three days the voltage on the board connector was still more than 200 Volts! So I have to state that the capacitor have very low self discharge current.
I connected the resistor directly to the board connector as I could not reach the dedicated contacts on the flash head.
What mode is the SB-600 set to? If it is set to "Slave", it will ignore the hot shoe of your trigger and wait until it senses a flash of light instead. You need to set the SB-600 to manual mode just as if it were attached directly to your camera's hot shoe so that it will respond to the trigger on the flash's hot foot.
I have personally pitted screwdrivers on (non-photoflash) electrolytic capacitors that had sat charged but idle for WEEKS.
Self discharge specifications of such capacitors will always be worst-acceptable numbers, since self discharge and leakage currents are usually considered undesirable to an engineer.
Never rely on a big capacitor being safe unless you ...
If the flash is set at 1/1 full power, then it's less than on second after you have pressed the test or shutter button.
Unless you are an engineer or good flash tech, then you use a resistor and short the capacitor to ground.
Seems like your lock button is broken, my SB600 resists movement until the button is depressed.
EDIT (note to self: test before you post), actually it resists being raised, it can be lowered without depressing the button. Quite stiff though, the Fong must be quite heavy.