Sometimes PPT shows a black or a white box instead of a movie when the path to the movie contains too much characters (e.g. when the files are deep down within a network structure).
Maybe try to
- copy both PPT file and movie file to desktop
- shorten name of movie file
- then insert desktop movie into desktop PPT file
- if necessary: move both files back to network structure
(just something you could try when other things don't work out ...)