Be sure to follow the installation and getting started procedures as instructed in this document from Statmodel.com
MAC OS X users should be able to use the shell. Especially:
See this link from IU too.
muCommander is pretty nice. Includes path copying, two-panel/two-folder view.
You will need a plain text editor. I do believe TextEdit is what ships with MAC OS X. This is what you should use to edit INP files.
I found this video and these instructions on youtube, using TextWrangler (web site says it is free 7/30/2011). Looks very handy, especially if you could figure out how to link to mpdemo.
Without the Mplus/GUI, the Mplus PLOT features are not accessible. But, you can get at them via R. See here. Apparently Windows users can make use of these functions too. Cool!