First of all you'll need to wait, as neither the BSG mod or their demo are released yet. Their forum is located
Here, and I assume that's where the download links will be put up upon release.
It's supposed to be released as a standalone total conversion, so all the media you'll need for the game will be included in the package they put up once they're done. So, the thing you'll need is a working mac installation as I'd hazard a guess that they'll distribute it with a windows executable - The actual game media will work regardless of platform. I'm not exactly a capacity on mac matters (that's be Taylor. I'm mr. clueless when it comes to macs less than 8 years old), but until he sees this you can try looking
Here for the most recent builds (mac versions included), and browse some of the threads in this forum as many common OSX questions have probably already been answered