Question on turning a avi to an mpeg

Several months ago, I used MainActor to put together several avi clips I took with my digital camera and put them into one mpeg. Now, I can't seem to do it. I'm wondering if I'm doing something wrong. I downloaded my avi from my camera and tried to place it in MainActor, but MA said it was not recognized. It plays fine on my system. Do I need to convert it to an mpeg first? If so, how do I do it again? Thanks for any help offered. I'm trying to get something for the grandparents. Tom