Under exposed pictures when using studio lighting with 5D

A black shadow at the bottom of the image is usually the result of using a shutter speed that is beyond the max flash sync speed of your camera. The max sync speed of the 5D and 20D is 1/200th. In a studio setting with wireless triggers there can be additional latency. Try using a shutter speed of 1/125th and see if that clears it up. You may be able to push it a little higher as pocket wizards are very good wireless triggers but don't use a shutter speed above 1/200th.