To upload creatures to the obelisk make sure they are close enough to the terminal.
It is as simple as moving your creatures and yourself onto the platform and accessing the terminal in the middle. The other main feature as of now of the obelisks is to upload or download dino and survivor data that can then be transferred to other Arks. The Bosses page under the Dinosaurs and Other Creatures section explains everything you need to know about the Broodmother. One of the main features of the obelisks right now is to summon the end game boss, the Broodmother.