I had exactly the same problem in loading files from by HF10 onto my iMac but I didn't have the same problem on my Macbook. So I tried to copy the clips from my Macbook onto the iMac but imovie on the iMac wouldn't recognise them ...
This suggested to me that the problem was with the Imac's version of imovie being unable to read the file types from the HF10.
I had, in parallel, been trying to fix an error shown when iMovie on the imac starts up :
"The QuickTime components necessary to view, edit, import, and export several kinds of movies are not installed. These components are included with the iMovie installer; please re-install"
The discussion and solution to this problem can be found under the "Missing QuickTime Components" discussion on this site however to save you time I have copied below fgalvani70's solution (Bravo !) which works perfectly ... both the missing component issue and the import issue are now perfectly resolved !!
1 - install software Pacifist (
http://www.charlessoft.com/) (it's shareware but you can use without problems)
2 - Insert Original Apple installation disk
iMovie '08 missed a Quicktime Component called: “AppleIntermediateCodec.component”. This component is necessary for importing movies in iMovie 08.
So, just let Pacifist read the Install DVD and extract only the ‘AppleIntermediateCodec.component’ to the default directory.
It's under Contents of BundledSoftware.mpkg -> Contents of AppleIntermediateCodec.pkg -> Library -> QuickTime
Select AppleIntermediateCodec.component and click Install