Canons are a bit "picky"...
connect direct to your Mac, no "hub", no daisy chaining, no other devices on firewire (e.g. ext. harddrive...)
and try different routines:
launch iM first, then switch on Canon...
or..
switch Canon on, then launch iM....
or
some recommend
push play manually at Canon, then hit import at iM.. (needes once fo recognizing cam, then all remote controls should be operable...)
last advice, to "reset" iM:
quit iM
search in user/library/preferences for
com.apple.iMovie.plist
or
com.apple.iMovie3.plist
drag (both) into trash bin, launch iM again..............