I\'m not aware of any binaries of Crystal Space built for the latest release (0.98) or what Planeshift uses (current CVS head)
crystal.sourceforge.net is the place to start for all things CS - it should have the link to the Sourceforge developer page which has the instructions for getting the source from CVS.
With the developer tools installed, you have all the necessary tools (gcc, cvs, maybe jam). You might need to get a few of the free libraries that CS uses (libpng, libogg, libvorbis, libode, libcal3d, etc) if you want to have any fun.
There are usually helpful people on #crystalspace, and most problems that you\'d run into aren\'t necessarily mac-specific. I\'m occasionally around there, here, or #planeshift.
Good luck!