Sorry for the confusion.
Currently, to the best of my knowledge, there is no build of Forge that runs native in OSX (anyone with contrary facts please correct me on this). My copies of Forge (I have the updated Rubicon version also) both run in OS9 "Classic" mode.
My question about Aleph One had to do with the shapes file you're using. If the version of Infinity that you are playing is the Aleph One version, then your shapes file will have the extension I mentioned. If that's not the case, try this:
Make sure you have a shapes file that works with an actual Marathon game. Copy the shapes file in your Infinity folder and paste it into the folder that Forge is in. If you already have a shapes file in the Forge folder, get rid of it first. If I remember right, Forge comes with a shapes file as part of the d/l package. For whatever reason, this may not be a workable file. I seem to recall having a similar problem when I first d/l Forge and the open source release of the trilogy.
: thanks for your time.
My pleasure. I just hope this is helpful.