Why would you want to be modeling for FS using a Mac? You can't run FS on a Mac, and even if you design something in a program for Mac, you won't be able to transfer it directly to FS since you would have to import it to Gmax or Max to convert it.
But as far as your question:
Blender is a 3D modeling program that is compatible with both Windows and Mac, and is capable of importing (and exporting) most types of 3D models. Unfortunately, you'll still run into the issue I outlined above: you won't be able to animate, texture or convert the model unless you utilize Gmax (free) or Max 3D (payware).