Hello I'm new here

I have been trying (and trying) to create Mpeg2 files with muxed ac.3 (Dolby 5.1 Surround) audio. I have a pretty compitent Mac Pro and Final Cut Studio 6 with Compressor2 (which won't do it). So I have downloaded ffmpegX in the hope that that will work but it fails with this in the process information:

INFO: [mplex] mplex version 1.9.0 (2.2.7 $Date: 2006/02/01 22:23:01 $)
INFO: [mplex] File /Volumes/E Drive/MASTERS/MPEG/Program Streams/file.m2v looks like an MPEG Video stream.
**ERROR: [mplex] File /Volumes/E Drive/MASTERS/MPEG/Program Streams/file.ac3 unrecogniseable!
**ERROR: [mplex] Unrecogniseable file(s)... exiting.

Does anyone have any ideas on what I can do? Thanks