Here's how you can get a Lume Fossil in Pokemon Black:
* Talk to the Professor: After you defeat the first gym leader in Nacrene City, you'll receive a call from the Professor. He'll ask you to go to the Reversal Mountain in the north of the region.
* Find the Fossil: Go to the Reversal Mountain. In one of the caves, you'll find a Lume Fossil.
Once you have the Lume Fossil, you can revive it at the Revive Fossil Lab in Nacrene City to obtain Archeops.