Downloading Youtube video via Keepvid.com

Is anyone familiar with doing that? I've downloaded a lot of stuff just fine via keepvid.com, but sometimes I get files that I can't open. Keepvid says to change the name of the file to add the extension .flv, but that doesn't always work. Tried Handbrake and importing into iMovie with these files as well, but no go.

Does anyone know how to solve that? thanks!!!