The Mechanist lives in the old robot repair center by the town's entrance. This place will be full of leveled robots, so come in with some decent weapons. If you have 75 points in Lockpick, then congratulations. You can just go down the first hallway and pick the elevator door. This will take you right up to the forge.

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If you don't have enough in Lockpick, then just go forward and into the security area. Kill the robots in here and move up to the control room. Keep moving into the next robot bay and kill the turrets. There are control panels to emit pulses, turn off the turrets, and generally help you. The problem is that you usually have to kill just about everything to get to them. If you're good at sneaking, feel free to try it.
Clearing out this second area should open up a path up. The entrance to the forge itself is hidden. You'll need to access the coffee maker to make the door appear. Then use this to get inside and visit The Mechanist.
If you already took care of the AntAgonizer, you can turn in her costume for Protectron's Gaze. This is a great laser pistol that basically acts like a laser shotgun. If you want the costume back, then you should be able to kill him after he hands you the weapon. Then you can get both costumes. You can also ally with him to get the AntAgonizer to storm in and get killed.
If you want good karma more than the pistol, then you can pass a speech check to convince him to take off the suit and drop the name. You can also use Black Widow or Child-at-Heart. He'll turn in his costume and walk away.