To set up your webcam and troubleshoot some common video issues in Skype for Mac, please check the following:

First off, check that you are using the most recent version of Skype for Mac which you can download from HERE

Also ensure, that your computer software is up-to-date.  Run Software Update by clicking the Apple logo on the menu bar and selecting “Software Update”. Additional hardware such as webcams, sound cards, etc. should also have their drivers updated. Please contact the individual manufacturers of your additional hardware for details on the most up to date drivers.

If your video camera does not have drivers for Mac OS X, please use the drivers available from HERE

Make sure that the correct video device (webcam) is selected in Skype, under Skype > Preferences > Video and also, in the same preference pane the checkbox ”Enable Skype video” , is checked. You can also reset your custom settings by deleting the file com.skype.skype.plist located  in User/Library/Preferences, re-launching skype will then search for the available video devices.

Finally, check that you have closed all other applications that might be using the webcam.
These include: Video editing software, instant messengers (e.g. iChat), browsers (e.g. Safari or Firefox). Simply restarting both Skype and Mac may also help.