How do you get the power armor in fallout 1?

If you are talking about the armour on the front of the game box, you have to do a mission called 'Helping the Outcasts'. Warning this quest may only be accessable on the download disk of the Game of the Year Edition. Click show map marker on your pipboy 3000 and it leads you to a metro. Venture through the metro tunnel and you will come out in front of a staircase. An outcast is just visible from your position. (The outcasts look just like Brotherhood of Steel Knights except their armour is red and black). WARNING! Do not walk up to them in Enclave armour. Greet the outcast nearest to you in any other armor. He will speak and ask for help. Help them with their mission which you will soon see comprises of simply shooting Super Mutants heads off. Push the Super Mutant resistance back making sure you dont shoot the outcasts as they will turn on you. Eventually you will get to the outcast base and you will be told to venture inside. Go down the lift. One of the members of the outcasts will ask for more help: They need your pipboy 3000 to open a faulty door that leads to their armory. (A gold mine of powerful weapons). You follow the member into a room with a simulation that you have to complete. This simulation is to long to explain how to do but it is relatively easy to complete. Once you have finished the simulation you will be led to their armory. Inside will be the power armor, the legendary Gauss rifle, a rocket launcher and a heap of grenades and ammo. As the member offers all of it to you, take it all. This is a long quest but is worth it for the power armor.