Okay, it looks like you have two separate questions here. One is how to get back in here, the other is how to proceed with the game.
This room is the entrance to the postmastery level. You can only go in after finding all secrets.
To get in: The post you have quoted has directions. The "
special square"
is
Click here to view the secret text
×between the northwest bush and the citizen standing near it.
To proceed with the game: since you mentioned needing a pass, what you want to do is enter the building here, the City Archives. If they're not giving you a pass, you might have restored to a point before you were told you need one. In that case, you want to go to where the Slayers are in 5N3W. Watch a scene there, then go to the Archives. Do not restore in between these steps!