PS CS4 error

 So I'm getting the "could not complete your request because it is not the right kind of document" error when I try to open a video to make a gif. 

Things I've tried:

• I've got the latest version of Quicktime
• I've converted the avi videos to mp4 using a program, then changed the file type to mov
• I've converted the newly converted mp4 files to mov files using a program
• They open if I do the "file > import > video frames to layers" method, but they won't open when I try "file > open"
• I've looked through the tags here (which is where I learnt to convert to mp4 and change to mov) but no joy :(

I would really appreciate any suggestions!