Check to make sure that your video cable is plugged into the rear video port. Also make sure that if your stereo has a reverse cable that it is connected to reverse power somewhere in your car. On my stereo I just bought some power cable from Autozone and ran it from the back of my stereo all the way through my car and spliced into my reverse lights. If you want to always be able to switch to your reverse feed then you will need to plug the video cable into the alternate video port. However if you are doing this then your stereo's parking cable will need to be bypassed or grounded. Otherwise you won't get a picture when you are not in park. This is similar to when you put a DVD in and take it out of park, your car should turn off the picture so that you can't watch a DVD while your driving.
Your question does not give enough details to answer. The connection is pretty straight forward. You connect the camera to power and you connect the video line to your radio. Of course your radio needs to have support for a backup camera.
On any radio set with video cable input connected from camera... There is usually an automatic swtching of the screen to show the video input from the rear of vehicle when you are on reverse. However some cheaper radios require you switch to a video screen manually by selecting the proper screen. Double check all required electrical connections and then check your manual for further directions of still not working and all connections were good. If not done... Make sure the video cable is connected to the radio.