What your looking for will be called "Operation Mode". This contains settings on how the router operates. Two of the main settings under this will be "AP mode", and "router mode". It is there that the unit will be configured to function as either an access point or a router.
Take a look at this FAQ page on Asus' website: ASUSTeK Computer Inc. -Support- FAQ RT-N56U
(Ignore the fact that the screen shots used on the above page are for the RT-N10+, a lower end router in the same family; the GUI on this router and the RT-N56U are the exact same)
This next page contains screen shots showing the GUI from the RT-N56U, and that it does have the same layout: Asus RT-N56U - System Setup - operating mode - Asus RT-N56U Dual-Band Gigabit Wireless-N Router Review - Softpedia
Its very simple to switch the device to "AP" mode, as you can see in the above links. I guess the only way you could prove that it is in AP mode is letting the floor coordinator log in to it themselves, and seeing that it really is in AP mode. There is a possibility because it has the ability to switch into router mode that they may not allow it to be used. If that becomes the case, we will need to find a different device. However, finding access points by themselves is harder now than it used to be. Most routers have the ability to function as access points, so dedicated access points aren't in as high of a demand. They just aren't quite as common as they used to be.
As to where you plug in the ethernet cable that is coming from the wall, it can go in any port. I would plug it into the WAN port because its easier to discern from the other ethernet cords that might be plugged in. Simply put, its easier to "tell at a glance" what the ethernet cord is for, and where the other end is plugged in.
Overall its a very simple procedure. As you can see in the screen shots and instructions on the top link, its as simple as selecting AP mode, applying the settings, and customizing your AP settings through the set up wizard. You just need to create your own SSID (wireless name that devices connect to), and the encryption (WPA2-AES; ideally).
The only problem with the USB ports is that they are only USB 2.0. If you have USB 3.0 drives, you won't get the benefit of 3.0 speeds. However, if your using 2.0 drives, it doesn't really matter. I haven't tried it myself, but I do believe they also support printer sharing. And if not now, most likely will in a future firmware update.
Let me know if you need any more help.