How do I contact the Brotherhood of Steel?

Solution 1:

You can only enter the Brotherhood of Steel bunker if you discovered a dead Brotherhood of Steel Paladin and recovered their password, if you have Veronica with you or if you are told to find them by either House or Yes Man. As a last resort you can also pick the very hard lock (100 skill required).

Entering the bunker for the first time always starts the Still in the Dark quest

See the Hidden Valley Bunker for more details.

Solution 2:

There are 4 bunkers in Hidden Valley. Just as you go past the gate for Hidden Valley, it's just to your left. It has a tree stump on top of it, the fence will be next to it too.

Once inside the first part of bunker, the door will be locked, needing a skill at 100. Or, an easy way to get inside at level 2 is to just go to town 188, pick up the bodyguard in there, then come back to Hidden Valley. She will open it for you quick and easier.

Oh, once most missions are done, the elder will tell you to come back later. That means you need to go out of the bunker and come back in 2-3 days' time. Then, more quests will open with both the elder and that paladin.

Solution 3:

There is another way to get in through the Hidden Valley that I accidentally stumbled upon:

I forgot which bunker it was, but it had the door with computer next to it. You go into the room while crouching. Hide behind the crates or boxes in the room and wait like 20 seconds. Then walk out and you will see 3 of them standing at the door. Go talk to them and you will be given some quest you have to do.