Why doesn't my webcam work?

So I have tested my cam, and it is compatible. However for some reason it won't work with Yahoo Messenger. I have done everything I possibly can think of... however I do not know how to change my firewall codes and I do not have the options to choose a webcam in my preferences. Can anyone help?Why doesn't my webcam work?You need to open messenger and click messenger name.Click on preferences,click on my web cam.When screen opens up click on file,click on preferences,click on camera source.There make sure your camera name shows in the camera slot.If not put it in.If you cant then you need to download the cam CD again.And only hook up the web cam usb cable when it is asked for.If your not getting a firewall error then that's not your problem.Hope this helps,if not email me for more.