I've run into a couple of issues with this app that I've traced back to problems with one of my USB ports. The right hand port on my Macbook Pro also powers the iSight camera. This causes any USB camera on that port to not work properly with this app and cause problems for other cameras trying to connect (even IP cameras not coming through USB.) If found this link
http://createdigitalmusic.com/2009/04/27/mac-usb-audio-woes-m-audio-says-avoid-the-left-hand-usb-port/ that talks about this at some length regarding audio devices. (Note: the title and early posts have the right/left reference backwards. The problem occurs on my 15" Macbook Pro with the USB port on the right side, not the side with the power connection.)
All laptops seem to share the problem of putting multiple devices on a single USB bus (bluetooth, built-in cameras, keyboards, trackpads, etc.) This appears to cause the port to deliver insufficient power to the device. Go to System Profiler on Mac (System Manager on Windows) and check our USB devices to see if you have more than one device showing up on a single USB bus. This may be the cause of problems with multiple USB cameras.
