I have recently tried to take pictures of stars using my wife's D7000, but I couldn't get it to expose for more than 30 sec. In my old D70 I used the wired remote to do long exposures, but now I have only the wireless one. How can I get it to work?
You need to set the shutter speed to Bulb, which can only be done in Manual mode (turn the mode dial to M).
Once you've done that, and set the camera to use the remote, simply press the remote button once to open the shutter, then again to close it.
It's worth noting that there is a 30 second limit on bulb mode when using the Nikon ML3 wireless remote. You need to use a cabled remote for longer exposures, for some reason.
Further to ElendilltheTrail, I would also consider putting the camera in mirror lockup mode to reduce the risk of camera shake. Also note, that if you expose the sensor for more than ~30 seconds the rotation of the earth will create star trails, unless you have some kind of celestial motion-control tracking rig. Good luck, I've tried this a few times without much success (but I live in NYC).
Follow these steps to take exposures over 30 seconds with the wireless remote (ML-L3) on the Nikon D7000:
1) Go to manual mode and select "bulb" for shutter speed.
2) Change the release mode to remote.
3) Go into the "Shooting Menu", select "Remote Control Mode" and note the setting.
4a) If in "Delayed remote" or "Quick-response remote" mode, begin the exposure by pressing the remote button once .
4b) If in "Remote mirror-up" mode, begin the exposure by pressing the remote button twice within 30 seconds. If the remote button is not pressed a second time within 30 seconds, an exposure with 1/4 second shutter speed will be taken (see page 81 of the manual).
5) Wait the desired time for the length of the exposure and press the remote button a final time. If the remote button is not pressed within 30 minutes, the exposure will automatically end to prevent overheating of the sensor.