I've been trying to get an external USB WiFi adapter to work with OS X 10.10 (Yosemite), instead of the internal one that came with my iMac, but I can't find anything in system preferences whatsoever that lets me configure it, I looked in network settings but couldn't find any option to select the adapter I want to use, it just lets me pick 'WiFi' which is always the internal one.

The adapter I'm trying to configure is an Alfa AWUS036NH, any help will be greatly appreciated!