Hi,
You need some sort of compositing software to load your rendered files into which can then export them as a movie. Some of the big ones out there are Fusion by eyeon software, Adobe AFter Effects, and Nuke (made by The Foundry i think) seems to be the next up and coming big one. You would want compositing software if you want to take a 3d element you made and the put it into an existing video/plate. Video editing software may be able to do this as well, such as Adobe Premiere. I believe that newer versions of photshop can even play an image sequence as a movie too. Also, the professional version of Quicktime can import an image sequence and play it as a movie, it may be able to save it as a .mov as well, but I cant remember for sure.
