FOR PC PLAYERS:
From the main menu:
1. select "single player"
2. select "new game"
3. select "dimension of the Past"
4. You need to jump through the normal portal. Then go behind the teleporter before you and to the left is a little passage. Follow that and now jump through the Nightmare portal. It will bring you back to the room you were just in, don't worry, its now set to nightmare level. Jump in the teleporter in front of you.
5. hit ~ to bring up console command
6. type "map e5end" (you have to start THIS campaign to be able to warp this level)
7. type "god" and "noclip" and fly to the exit portal. You will see QUAKE written above it. I had to turn off no clip and kill the enemies in the room for the barrier to go down. Jump and the achievement is yours.