This is one from work. We use Avid, a lot. The problem is it’s really finiky about what you can push in to it. One free and fast work around is to use avidemux to re-encode or rewrap your video so it can read it.
Grab Avidemux from http://avidemux.sf.net
Fire it up, open you file, let’s assume it’s an MP4 or FLV from youtube. Click Next/Yes to use Safe Mode on the video if asked.
Now, just use the following settings on the left to export:
Video Codec = MPEG4-AVC / Audio Codec = AC3 (lavc) / Container = MP4
Hit save video. Wait a bit and tada, an MP4 that can be imported in to Avid Media Composer 5+.