ahhh... okay i understand. (i think)
so the reader with the card in should show up as a removable drive on your desktop.. right? No?
if so, try moving the movie file directly from the removable drive on your desktop to your desktop.. (does that make sense?)
then try to open the file NOT from the removable drive (card reader) but from the file you just transfered on your desktop.
if NOT, then i'd say that there is something wrong with the reader.
another thought is that because you are using a memory stick (sony) that it maybe in a PC format, and for some reason you might need to add an extension to the end (ie .dv, or something else - depends on what your camera saves video as) BUT this doesn't matter if you can't see the drive at all...
edit:
AND IF none of this makes sense or doesn't work, why don't you just transfer the movie/photos directly from your camera via USB/Firewire (if imacs have firewire)??
let us know if any of that helps.