I'm not sure what ACS Defend does... and how to use it. I clicked it for my buddy's planet and then it asked me if i wanted to transport stuff or attack it... and I just wanted to defend it?

Make sure your friend has an ACS Depot.

EDIT: I have just tried this with a friend who has an ACS Depot and i got the same results as you. This should probably be moved to the Bugs/Typos forum.

-Istalris-

What exactly does an ACS Depo do? Is it just there so I can send my fleet to defend it?

The ACS Depot supplies fuel (Deuterium) to orbiting friendly ships. This allows a player to ACS defend your planet, the Deuterium used to fuel the defending ships will come from the defended planets stockpile. There is a limit to the amount of fuel that can be supplied (and so the size of the defending fleet) by the ACS Depot's level. At level 1 the depot can supply up to 10,000 units of Deuterium fuel per hour, this will increase with each level.

-Istalris-