Oddly enough, I was just looking for iMovie 6 for an older computer on the internet yesterday, and found it in one of the usual haunts. You should be able to run iMovie 6 without having to delete the current iMovie. When Apple introduced the new version, iMovie 6 was just moved to a folder identifying it as the previous version, and you can run it from that. If you have it on your iMac, you can simply copy the app over to your Macbook. If not, I would reinstall the app from your Time Machine to the iMac and then copy it over to the Macbook. I haven't tried iMovie 6 under anything more recent than Leopard, though: does it run under Lion?
2.9 i7 iMac; Mini i5 2.5; G4 Cube!; iPad 3; ATV 3