What is an "MPEG member"? Exactly what type of member are you talking about?
The only video member type that permits masking is #quickTimeMedia, so if you create a QuiickTime member and link it to your MPEG video you can set member.mask
Its a MPEG-1 Video member. So,how can I link it to a QT Media??
Thanks for your help.
Insert -> Media Element -> QuickTime
Name the empty QT member that this creates and open (if it isn't already) the Property Inspector to the 'Member' tab. Click the button labelled "..." at the bottom-right of the Filename text entry field and select your MPEG file.
Read the docs for member.mask