I´m also struggling with this problem for quite some weeks now.
What I found so far is, that the new GStreamer video library lacks capability to work with video grabbing USB devices. It seems unable to select the desired input (composite / S-Video) and or the PAL/NTSC.
Correct me if I´m wrong, I would highly appreciate a way to get an USB video grabber running in Processing 2.0
An workaround is to switch back to Processing 1.5.1. which relies on the old video implementation using Quicktime. That works for me, but all the other code that I migrated to 2.0 is now useless.