Hello
Well I found your forums seeking out info on the same thing, MP4 to MPEG and followed all instructions over at ffmpegx and just in case someone needs this answer later on here's how to do it.
Go to ffmpegx.com
You'll see text at the top of that page instructing you to download mencoder and mplayer binaries; these are both engines. Next when you click on the mpeg2enc binary which is also needed as per the instructions, you'll have a massive page of binary code before you; copy this and save it as a text document and simply call it mpeg2.
Make sure to also download the newest client of ffmpegx and just unpack it like any other Mac program, open it, then you'll see three sections in red, one for the mencoder, mplayer binaries and mpeg2enc binary, find them on your Mac and assert them to the proper path and that's it.
They did try offering me a program called Rosetta, so I denied it and ffpegx was all set to go.
I hope this helps.