I don't work on Macs very often, and when I do it's generally very simple stuff (like this). But I don't do it often enough to quote the process by heart. So maybe you should wait for a Mac guy to respond.
Thet being said, first make sure the Airport card is turned on. There should be a little antenna icon on the top for the AirPort connection. (There may also be a physical switch somewhere on the laptop, although frankly I don't remember ever seeing one on a Mac).
Once you make sure the Airport card is turned on, somewhere in there you should find a function to scan for networks; and once it sees the network, there should be a function to connect to it.
I don't remember ever having to connect a Mac to a non-secured network. Like I said, I rarely work on them. But maybe you have to manually authorize a Mac to connect to a non-secure network, like you do with Window Wireless Zero.
Again, I'm no Mac expert; so take the above for what it's worth. But it's something you can try in the meantime until someone who knows what s/he is talking about responds.
-Rich