Apple's Mac Pro Card
If it's one of the newest Mac Pros, and possibly any Mac Pro, you can't put the old style Airport Extreme card in. It's not the same plug and play deal anymore unfortunately.
I just went through this this weekend actually, wanting Airport in my Mac Pro. Maybe someone else can post an alternative, but if you decide to, you can go the Apple route and purchase an Airport upgrade kit. It's a small card that has to be put in by an authorized Apple service provider, but honestly, if you're brave and willing to risk possible warranty disruption, it's a breeze to put in yourself. If you do, you would need to order the kits from Apple. My local Apple store had no idea they even sold them! The part number for the kit is MB363Z/A for the early 2008 Mac Pro models, and MA688Z/B for the earlier Mac Pro models, and it cost $43. Not too bad for the 'official' Apple Airport. Although, really, it should come standard these days.