To find Area 51, travel down to the Mountain States region. The base is right on the edge of the Mojave Desert, right as you enter the Western Shrub-Steppes.

One way to find the base is to start at the "Western Shrub-Steppes" title on the map and travel southeast. The big silver UFO is walled-off inside the army base.

There doesn't seem to be any easy way inside, but the UFO is clearly visible through the fences. Go into first-person view and look through the uncovered exterior to get a good look at this alien oddity.