You would have to go to the museum in Pewter City and talk to some scientist all the way to the right. He'll revive to fossil as soon as you leave the building.
Where Do I Take The Fossils In Pokemon Heart/gold And Soulsilver?
You have to get to pewter city where Brock's gym is, where the museum is or used to be, enter and go all the way to the right the man will revive the fossils.