I do not know how to install the ASUS USB-AC51 wireless adapter. I am not very good with
There is a newer Linux driver for the underlying chipset "MT7610U USB" here: https://github.com/maage/mt7610u_wifi/tree/some_changes
This worked for me on Ubuntu 14.04.4, whereas the MediaTek drivers did not. The above is a fork of the MediaTek drivers which fixes the compilation errors and warnings.
I bought this adapter yesterday, and at first it seemed like nothing helps to make it work.
Luckily, I found this: TP-Link AC600 driver on Ubuntu 14.04 LTS x64
P.S. It is a guide for TP-Link's driver, but it seems to do the trick for ASUS USB-AC51.