If you google MPEG Stream clip, download that program and convert you footage to something FCE can understand. H.24 (your gopro footy) is basically a MP4 codec and is only meant to be a finishing codec, so editing programs dont usually accept them. I believe, apple intermediate codec might work well you, its on the larger file size but FCE doesn't give you a lot of options.
I actually have a workflow written down for a flip cam workflow, but its the same thing. if you PM me i could send it your way.
Imovie works if you just want to upload to internet, if you want to actually edit i would use FCE.