How can I get my camera to work?

I am new to Unity an have been trying to get started with AR using vuforia. Have ben trying to follow a youtube tutorial and I can set up the AR camera, load the tracker image etc. but when I press play to test things, all I get is a black screen and my camera light does not come on. I am using a macbook pro running Yosemite. The camera is the built in FaceTime HD. I the Vuforia settings it shows that my camera is recognised. Is this a unity issue? a vuforia issue? a compatibility issue or me missing something basic (highly likely!)

Any help appreciated!