Going off of memory, and based on your description (it wasn't exactly detailed), it sounds like you didn't have the discussion with the elders?
Well, whatever the case is, you need to have your evidence and present it to the elders, they'll have their meeting, of which you'll sit in, and you'll offer your evidence to them, and then you'll be notified that the BOS will send forces to back you up at Mariposa. They will just be there when you arrive.
HOWEVER, all they'll do is send 3 heavily armed Paladins to the outdoor map, and they will join you in combat. They will NOT follow you inside. No matter how you engage the guards outside Mariposa, the base will ALWAYS be alerted to your presence if you take more than 3 turns to kill every Super Mutant outside, so the 3 Paladins help finish the job in fewer combat turns, but so does having a REALLY effective character with 10 AP, a Turbo Plasma Rifle, Fast Shot, and Bonus Rate of Fire.
If you're pretty weak, the Paladins will help you get inside the base, but they won't be of much use to you inside, because you'll need to know how to get around the inside of the base on your own. If you can't handle 4 Super Mutants in a wide open space, you'll be in for a world of trouble facing off against 5 or more in close quarters...
