sounds like u have a none itunes plus video, meaning it has drm and can't be played in serato. some will tell u to use a program to screen capture the video which is basically re-recording it, but u lose a generation of video quality.

when u buyem make sure they're itunes plus to play using serato.

you can try using a program called Tunebite. It takes the DRM protection off of vids. I'm still trying to find a MAC friendly program that will work.

i just had this issue...

what format are the videos?
also, we're assuming you got the videos from itunes which may not be correct.

were the videos from itunes?