Following bisjac, I'm on a Mac. And I've been hosting servers since 0002 (used WINE or Crossovers back then).
You just need to port forward your router. You can do this by logging into your router (eg. 192.168.0.1), and assigning the following ports for Blockland. Badspot setup a step-by-step on what ports to use. Do a search.
And for all those people saying that Macs suck: Mine runs four gig's of hardly used RAM, and has never gotten a virus in it's life; I can run Garry's Mod at 100 frames-per-second.