If you want to view multiple cameras connected to the same computer, you just need to run multiple instances of iCamSource, one for each camera.
To do that on a Mac, you'll need to make a copy of the iCamSource application for each camera. First, run the first iCamSource and set up the first camera. Then run the second copy of iCamSource and set up the second camera. Make sure that all instances of iCamSource have the exact same login and password entered.