i've killed all the mutants outside the military base and used the radio to call help. but no more mutants come, and i'm stuck and can't get in the base.

To get in the base you either have to pick the lock on the door, or the mutant standing closest to the door should have dropped a tape with a passcode to the door on it.

I've had it that the disk with the code on it doesn't spawn. What you can try is using a radio as soon as you enter the area to fake a distress call causing the 3 out of the 4 mutants to leave, and then killing the remaining one quickly. He should drop the keycard after this.

That didn't work for me. I also tried killing them all and spent an hour looking for the Thieves' Circle to find an electric lockpick to no avail.

Sorry this is seven years late XD I'm really desperate and I can't find anywhere else talking about it

I'm currently playing Fallout and entered the base yesterday. I killed all four mutants and searched each of the fallen bodies. On one of the bodies I found a holodisk. Reading the holodisk gave my character the key code to the base door lock. Then it was just a case of clicking on the door to open it.

If you still have the save, I can help you.
Upload the savegame folder (in a .zip file).
I'll send you the save, with the problem solved, and you can keep playing.