http://www.mediatek.com/en/downloads/pciert2790rt309xrt35x2rt539xrt3290mt7630/
If case the link above is old or stale, I searched for "RT539" on mediatek website and then had to enter my email address and submit. Then in less that 1 hour, I got a reply back from Mediatek support with a URL where I could download their driver. My RT5390 worked right away. I am now getting 72Mbps at radio type: 802.11n.