Welcome ,I will give a full and clear answer to your question, please make sure to reply to any questions I have. They may be essential to my reply.
Hi , what version of Imovie do you use?
Importing formats do differ on versions of imovie .
perhaps the codec within the movie is not supported .
Please try Mpeg stream clip to convert your video http://www.squared5.com/
MPEG Streamclip can export all these formats to QuickTime, DV/DV50, AVI/DivX and MPEG-4 with high quality encoding and even uncompressed or HD video.
The best format for use with imovie is DV or DVpro that is uncompressed , you can also use Quicktime movie / MP4 with H.264 AAC audio with a high bit rate this can also be effective . DV video footage will take more than x5 more storage space that a H.264 encoded video file. Files maybe converted to imovies own working format once imported , normally DV as provided from most camcorders .
Depending on the imovie version if more recent try the Apple Intermediate Codec.
To do this , open stream clip ,
See file menu ,
See export options ,
See Export to Quicktime
See Top most compression selection menu , Select Apple Intermediate Codec
Once selected , press make movie .
Name your file , and choose where on your Mac to export .
And press save .
The file should be able to be read / imported by imovie .
I hope this helps.
Sorry, the elections took all of my attention. Thank you so much for your help!