Give Micro Fusion Cells to Chris to join the Enclave faction
Go to Navarro and talk to Chris, the guy in purple robe. He will ask you to to bring him 60 Micro Fusion Cells. After giving him what he wants, take the little path above between the trees and use the computer in the booth in front of the base entrance to confirm your membership in the Enclave faction.
After the Enclave base was destroyed, there is probably no way to complete this quest.