Beldum:
Because Wallace is the champion, you must fight Steven. Steven is inside Meteor Falls.
When you enter, go up the waterfall and into the door. Next, go down the ladder, then go
to the ladder in the south. Now, go inside the nearby door. Steven should be to the right.
REMEMBER: If you are going to battle him, you can defeat him with Water, Ice (for
Cradily), and Fire types for most of his Pokemon. He has Pokemon over level 70. .After you
defeat him, go to his house in Mossdeep to get the Beldum.

He's right! ( and took my answer... )

in steven's house after the elite four is beaten.

go to stevens house on mosdeep city after you beat the elite four