how to access the unique rooms
For unlocking - once you've gotten the ability to place decorations and increased your inn level, talk to Rei (the fairy) and ask her about more improvements to the inn. Then you'll get a quest to go talk to the dwarf in town.
To build - When you open the room building menu, there will be a new tab at the top, next to "room" and "hallway" that says "special rooms". Click it, and you'll be able to build any that you have unlocked.