Connect the USB cable attached to the external webcam to an available USB port on the computer. Allow 10-15 seconds for the computer to recognize the external webcam.
Select the "Photo Booth" program, a program that uses the webcam to show you live video of yourself, from the applications dock at the bottom of the screen. Select "Camera" from the options at the top of the screen.
Select the external USB webcam from the list of options from the menu. The computer will switch from the default iSight webcam to the external webcam. Because Mac programs are all in sync with one another, these settings will be used for all other installed programs on the computer.