There are a few things to try:

Check that the files are on the supported file type list.

Check that the codecs inside the files are supported by your system.

A quick way to do this is to open them in quicktime player and see if they play. You can also use the "get info" command in the file menu in quicktime to see details of the codecs inside the files.

Codecs supported by Serato Video are H.264, DV, Motion JPEG A, Motion JPEG B, MPEG-4. You can get more information on codecs here.