The video files from your 5D mk2 are H.264 files that Premiere prior to CS5 can’t edit natively. I have the same problem with footage from my 550D/T2i. The solution, other than upgrading to CS5, is to convert those files into a format that Premiere can play with. I’ve been using Cineform’s NeoScene to do that, and so far I’ve been very happy with it. It converts the original video files to Cineform .avi files that Premiere CS4 has no problem working with and costs about $130. There are some other conversion tools out there that do a similar job, bit I don’t know how well they work.