When XCOM: Enemy Unknown Elite Edition - the Mac exclusive port of XCOM that contains all the DLC - launched on April 25th, the one place it couldn't be obtained was Steam. Apparently Feral Interactive, the company behind quite a few mac ports of AAA games, including the Arkham and Bioshock games, doesn't release games on Steam, and currently have no plans to do so.

If you're someone who was looking for the Mac version of XCOM, you can find a list of places that do sell it here. For the rest of us, feel free to just grab it on Steam if you don't already have it.