Instructions
Head out of Jubilife City on Route 203. This will lead you to the Oreburgh Gate, which is really a small cave. Go through it, battling the two trainers along the way.
Enter Oreburgh City. Head north until you come across the Oreburgh Mining Museum; the building will be marked with a sign designating it as such. Check the exhibits if you wish, then head to the back of the building and up the stairs.
Speak to the man in the back room. He will explain to you that he and his fellow researchers have developed the technology to revive the fossils of long-extinct Pokemon. If you have any fossils, you can hand one over to be revived. Come back in a couple hours to pick up your new Pokemon.