Put on a suit and send it to King Leorik's House in A 1. After entering the door, go right. There is a 2.4 newly opened map. The royal mausoleum seems to be called, and there is a room full of books in it. There is a table in it, which is directly called the suit dungeon. Click on it and put down a page. Look at the description on the page, it will tell you where the entrance to the dungeon is. Just take the page to the designated place.
The entrance to every suite is not allowed. If you need a specific location, you can go to diablo 3 District of NGA. There are 24 sets of underground city entrance maps on the homepage.