How do I install and play the Wilderness Missions? The Incubation installer will only install the base game. I can't find a download link for the Wilderness Missions.

The Wilderness Missions are installed along the base game, there is no separated installer for it.

Once installed there is two shortcuts in the start menu group for incubation to launch the game (at least in Windows 7) one for the base game an another for the expansion.
If you are using galaxy you’ll have to click in the “More” button in the Incubation page, select “other” and then you’ll see the option to launch the Wilderness Missions