According
http://wiki.mikrotik.com/wiki/Supported_Hardware#
Ubiquiti WiFiStation USB card (AR9271) works on N but causes reboot on G (ROS 5.8 x86)
What is for RB751U-2HnD?
Will I get an additional physical wireless interface?
What wireless USB adapters are compatible with the RB751U-2HnD?
TP-Link TL-WN822N?
None of the USB wireless cards are very stable, but you can try. Yes, you will just see one additional wireless card in interfaces. We have not tested anything else than what’s in the Wiki
ROS driver for AR9271 wireless USB cards ar not stable 
The are several threads on this issue I hope in the next future to have good news 
Giuseppe
Testing with RouterOS version 5.11 the rb751 with a tp-link AR9271 not get it to work, the log provides the wireless connection is established but does not work i tested bridging and routing.
i have the same question. mine is BlueLink BL-U93N though, i believe it uses Realtek chipset.
when i
/sys resource usb print
the result is
# DEVICE VENDOR NAME SPEED
5 1:2 Ralink 802.11 n WLAN 480 Mbps
but there is nothing on
/port print
and also none to
/interface wireless print
is that mean the adapter is not supported? is there any guide/wiki for usb wireless adapters? i can’t find any on the wiki, except for USB 3G modem. i also goggled it, apparently i can only find this thread 
tested on v5.8 ROS x86. i won’t dare to test v5.12 ROS yet(after getting a lot of crashes on previous v5.11 update)