You have to go to Steven's house to get it, he doesn't give it to you.
Fly/Soar to Mossdeep City, go north, then west. You'll find a house at the edge. Enter it, and there will be a letter from Steven and a Pokéball containing Beldum. It is level one, has the ability Clear Body and holds an Iron Plate, which can be used to change the forme of Arceus as well as boost the power of moves. It knows Take Down and it will have your OT.
If youre bored you can interact with his rock collection from various caves in different regions, but there isnt a purpose.