-It Is Highly Recomended To Do This With A Freind

-First you go to the map "LastOutpost"
-Ones your in a game on the "LastOutpost" Map In chat type " alien"
-Then wait
-After your turn is over, an alien should spawn
-After the alien spawns kill it and you should unlock the Area 51 Map

If these instructions are unclear here is an video explaning it