rmvb IS a video file (real media variable bit rate). I’m guessing your problem is one of these two:
1. You can’t play the file. Get real player or real alternative (recommended). http://home.hccnet.nl/h.edskes/finalbuilds.htm (scroll down)
2. You can play the file, but you want to convert it to a different format. Real Media is a proprietary format and I can’t think of any program that will convert it to another format off the top of my head. (If you find one, post in thread 3459; the guy there is looking for such a tool too. http://www.moviecodec.com/thread/3459.html)
rmvb video can be played by realone player.
If you want to convert video format on Mac, you can turn to mac video converter.
Hope it helps.