This is by far the most serious problem with iMovie 10. Apple has removed support for import of iMovie mobile projects which was the key to my workflow. I have discussed this with a senior tech at Apple and they have referred the matter to Apple software engineering, but of course they could not tell me if this function is slated to be included in a future update of iMovie 10.
I have reverted back to iMovie 9 on the Mac and iMovie 1.42 for iOS. Unfortunately you cannot use the new iOS version of iMovie (ver 2.0) because a imoviemobile project exported by the new iMovie for iOS will NOT import into the old Mac version of iMovie.
Furthermore, you cannot downgrade to an older version of an iOS application using the App Store. Your only hope is to find a copy of iMovie.app in a backup of your iTunes media folder from before the update. Delete the new iMovie for iOS from both iTunes and your iPhone or iPad. On your I-device, turn off automatic app updates (and do the same on your Mac).
Now find the old iMovie for iOS file on your backup drive (hopefully you have this on your time machine) and just double-click the file name. This will add the app to iTunes on your Mac. Now connect your iPhone or iPad and sync this app to your I-device. Now you should be back in business.
Good luck.