Now, activate the PaP opening sequence, as you would normally do it. The wall behind the machine will be gone, revealing an altar with RICHTOFEN writen in it's front, and the Focusing stone hovering above it. Pick it up to complete the easter egg. Congratulations !
(Note that only the one picking up the stone will have the perma perks, but everyone will have the achievement.)
Once the easter egg is done, you can do it again in the same game, in order to get more focusing stones. To do so, simply interact with the radio that appeared in the spawning area, beneath Pack-a-Punch stairs.