National Guard Armory

Its kind of hard to explain.. But if you found the training range or "shooting range" there is a doorway on the otherside of that room that will lead you up stairs, it's kind of hard to see.

You may want to try the walkthrough section of this site. Also I know that there is a video of this area that shows how to get to the switch you may want to search for that.

The Fallout 3 wiki page can help you here, and so can I, but it's really difficult to explain.

First off, pack your crap for this trip, because there will be a boat load of robots waiting for you. This means Pulse Grenades, Lasers Weapons, and Plasma Weapons. (I like A3-21's Plasma Rifle here, as well as a healthy dose of Pulse Grenades for the Sentry Bots.) Having an ally would be good too, maybe Fawks and Dogmeat (Sergeant RL3 isn't too good here because he could get hit by your pulse grenades)

Now, head inside.If you take the staircase upwards to your left, you should be able to jump a gap and jump a pipe right at the edge of the ledge. Now, simply continue on through a room with a couple of cubicles. Now, continue on until you find a hole in the floor. Drop down, and continue on until you find the cafeteria (Two nuka-cola machines are in it) you should find a door to the next area. From here, it's pretty straight forward. Once you get to the firing range, pick up all of the weapons and ammo while you're there, there's quite a bit. One in the firing range, take aim at the three or so robots downrange and open fire. Continue to the offices. Be ready to fight some Mr. Gusty's,, Protectotrons, Turrets, and a Sentry bot (Normal Difficulty) plow through and use a few Pulse Grenades to take out the sentry bot. The door behind him should lead back to the first area, but this time, you'll be looking out over the ground floor. There should be a switch, so pull it and look for the yellow lights and toss a couple pulse grenades in the door to take care of the Sentry Bot that tries to come out. From here, just flip one more switch and head into the armory. Note that everything in here is of pretty poor condition. Either have a good repair skill yourself, or have a vender do it for you. There is also the Small Guns bobble head, so make sure to pick that up. Lastly, you should note a large metal door that you can't get through. To access this door, find the 5 Keller Family Transcripts scattered across the wastes and listen to them all to learn the pass code. Now, open the door. (CAUTION: There is one Glowing One in the room, so make sure you're ready to kill it) inside there is 5 mini nukes and the most powerful weapon in the game (Of greater or equal power to the Mysterious Stranger's Pistol which is only available via console cheats): The Experimental MIRV. You need 8 mini nukes to fire it once, and it fires 8 mini nukes like a shotgun blast. Great for taking out Super Mutant Behemoth's or just screwing around.

I don't believe you can get up there but the door does open just get all the keller family recordings(5)