Install Perian and it will make Quicktime recognise .flv and you should then be able to import it into iMovie.
VisualHub baby!
Convert it to .mp4. Should work then.
Are you using iMovie 08? It's rubbish! 07 is a much better program.
Also, must be free - didnt mention that my bad...
Well you shouldn't have spent so much on the new MacBook.