Hi , I would like to help.
If there are problems with the import , it maybe best to bring in the media to the Mac desktop , then import .
Drag the video files from the SD card etc to the Mac desktop .
Try importing the files in to imovie .
If there is a fail , try playing the Files with VLC to test ok . And then convert the files to a new file such as MP4 or DV video to import in to imovie as this may fix a problem in doing so .
If your using an older version of imovie problems may be compounded , many issues with import export have been fixed in later 09 and now the new imovie 11 , in ilife 011 imovie it is far better than the imovie version from ilife 08 .
Imovie 6 HD was ok and plugging for effects , imovie 7 is bad , imovie 08 is borderline , movie 09 is ok , imovie 011 is best so far . To give you an idea .
VLC player http://www.videolan.org/
Video converter MPEG stream clip : http://www.squared5.com/
You can also try this more simple step to try to correct some Mac OSX issues :-Turn off the Mac .Power on the Mac .Hold shift on the keyboard using a USB keyboard , while powering on the Mac and booting , boot to safe mode.Once booted to safe mode , restart and boot as normal .Full details here :-http://support.apple.com/kb/ht1455This may fix minor issues but it is best to run a Full Mac HD repair from the OSX DVD installer disk , use the OSX disk that goes with your current OSX install ..Once the Mac HD is tested ok or repaired , you can go on to update the software , please let me know if there are issues ,and give full details. Please let me know if this helps , Thanks .