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As mentioned in the Vault 13 area, Goris in Vault 13 is the only one who can add this area to your World Map, though you can simply walk around the World Map until you pass its unidentified green circle and enter that way.
The Military Base is full of super mutants and your party members can easily get themselves in a bind, so leave them at the door if you're unsure of their abilities.
When you enter, loot the area including the NE tent with the Survey Map to San Francisco (you need to Use it), and kill the wolves who will attack. Finding the actual entrance into the base from this area is simple. Pick up the metal pole from the south sidewalk, attach it to the cart, find the dynamite in a nearby shed and use it on the cart (not arming it first). This will blow a hole in the rocks and give you a nice bit of xp.
On Level 1, deal with the radiation rats and a mutant named Grundel, then find the generator in the west. Repair it (and use a tool to help if you need it) to restore power to the facility, namely the elevator in the northwest corner. Take the elevator down to level 2 for some super mutant fun.
On Level 2, it's time to take down the muties. You'll fight a rather big group of them so be forewarned. Be sure to grab the Power Armor in the room with the lockers, and the super stimpaks in the wall locker on the northeast end. Proceed to level 3 for a slightly toughter fight when you're thoroughly satisfied with your work.
On Level 3, be prepared to fight muties with long range weapons... which can get a bit hairy when you're trying to deal with one at a time. Use the walls and corners to your advantage. You'll find an important Green Memory Module in a locker on the southeast end of the level. Use the elevator in the same section to head down to level 4.
On Level 4, when you first enter, there's a footlocker below the bed in the room nearby. Search that then proceed north for a tough battle. Save your game just in case. This is Melchior, a mutie magician of sorts. The result of this encounter will end in either his death or yours, and he's pretty tough, so be careful. He's a summon master and will continuously unleash enemies (about 18 of them in total) unless you take him out... but he does have a limit. If you feel powerful enough and/or want the most xp, continue taking out the deathclaws, floaters, geckos and mole rats until you feel satisfied, and then take on Melchior. Pick up the gauss pistol in the crate nearby and you're done here. Now on to SF if you've been following the walkthrough.