Scroll way down to the bottom of the page and read under section option 2.
The driver is built into the system already. Here's what I would do. Boot the system with the usb wireless unplugged, login and open a command prompt (terminal). Plug in the adapter, wait 30 seconds and then type 'sudo dmesg' it may ask for your root password or not. It will scroll alot of information, but we're only interested in the last handful of lines, it SHOULD say that it detected the device plugged in and setup. Next type 'sudo ifconfig' and you should have several entries, you should have one that says wan0, and it should have an IP address.
Additionally, I recommend you activate DHCP before these steps, that way it will connect and get the proper addresses.
Traditionally USB based wireless has been problematic in linux. The support is there, but with such a vast number of adapters using the same chips, each has the potential to require minor driver variations, and thats just too much for a volunteer effort to support.