How did this get resolved? I am having an identical issue.
Thank you for your post. As we have advised anniekuo via email, for a Built-In/USB camera, you'll need to select the "USB /Built-In / Firewire" tab in iCamSource and select the webcam from the Video drop down list. If your camera doesn't show up in the Video drop down list in iCamSource, please let me know what entries do show up in the list.
Also, please make sure there aren't any other video capture programs (such as Skype) running. They might be fighting iCamSource for access to the camera.
If that does not work, close the iCamSource application and then delete your com.skjm.icamsource.plist file in your Library/Preferences folder in your home folder. Then, when you re-start the iCamSource, you will be starting with a fresh set of preferences, and hopefully things will act as expected.
If you are running Lion it hides the ~/Library directory for some reason. You can get to that directory by running Finder and then holding the Option key while clicking on the "Go" menu at the top of the screen. If you're holding down the Option key, "Library" will be one of the choices available. You should choose Library, then open the Preferences folder. The plist file will be in there.