umm...

Well, whenever you receive a fossil from someone as a gift, or dig one up in the Sinnoh Underground, you are supposed to take it to a special person so that they can revive the fossil back into a fossil. If you want to have your fossils revived, then you should go see this person where he lives. In order to reach him, you need to head over to Oreburgh City and take a look inside the Rock Museum. Go inside and you should see some people behind a counter. Talk to one of them and a person will offer to revive your fossils. Say yes to the person, and then walk outside of the Museum. Go back in, and your fossil will be revived back into a Pokemon.

Hope this helps.

A fossil is just a free Pokemon inside a rock that requires some "special tratment".

Take a fossil once you get it to the rock museum in Oreburgh City, and talk to the person at the front desk. He will say "Come get it later", so you will decide to walk around for a while and go back in.

Easy way: Walk out, walk in.

He will give you the Pokemon, most likely rare, at a pretty low level. Keep it in your party and train it, and usually those Pokemon become strong.

You got a fossil from the underground so now you go to the Museum In orebourg. There should be a scientis behind the counter. Talk ti him and give him the fossil. Then step ouside and come back with an empty slot for a pokemon. Talk to the scientisst and he will give you a poemon.