Make sure that you install the quicktime component in before running macam - you MUST drag that component into the bookmark on the right. The macam application may work on its own before you drag-and-drop the component in, but believe me, you will really need this component to make it visible to other programs!
Plug in the PS3 Eye. When you open up macam you should see this blank screen with the words beneath it saying that it has been connected to your camera.
When you press the blue play button on the top left, you will see the webcam image. You can press play and toggle the image with the pause button. The other buttons allow you to save a photo or a video from within the macam application.
You can read more on macam in my previous project blog on building a tangible touch table with the ps3 eye. Installing macam will make your USB webcam (in this case, the PS3 Eye) available to reactivision.