1. Set the remote shooting mode to Quick response shooting or Delayed remote. Since the remote must be in front of the camera and within approximately 5 meters, you should probably use Delayed remote to allow time to remove it from view. The longer your exposure, the less this will be an issue.
2. With the camera set to M mode, select "--" as the shutter speed.
3. Press the shutter release once on the ML-L3 remote to open the shutter. If you selected Delayed mode, the shutter will open 2 seconds after you pressed the button. The shutter will remain open when you release the button.
4. Press the shutter release a second time to close the shutter. The maximum exposure time using the ML-L3 remote is 30 minutes. If you have not pressed the button a second time the shutter will close anyway after 30 minutes.

You can view the full instructions for the ML-L3 here.

I've found when taking long exposures that using a wired remote shutter release works better for me. You can operate the shutter from behind the camera and also have the ability to do a half press before releasing the shutter. You can use the viewfinder or the LCD screen in Live View to be sure of the framing and focus. For your Nikon D7000 you can use either the Nikon MC-DC2 or a generic equivalent. The wired remotes have a lock that holds the button down without having to physically hold it with your finger. I loop my wired remote around one of the adjustment knobs on my tripod so that I can let it go without it pulling on the camera.