Oh, wait. You need two empty build places on earth. When you are at tech level 5, after bringing the ZPM to earth, you should be able to build the Antarctica outpost and then the Atlantis Expedition. Then you have access to the midway station.
PS: they don't like double posts here, so please use the "edit" button...
"The way I see it, every life is a pile of
good things and bad things
.
The good things don’t always soften the bad things, but vice versa the bad things don’t always spoil the good things and make them unimportant."
- The Doctor