Short answer you can't. Your board is older and even though it has 2 usb 3 ports, back then there was no standard yet for an internal usb3 header. Therefore your motherboard only has the older usb2 headers and none for usb3 (didn't exist afterall).
What many people did instead, and many case manufactures did, was use a ghetto approach and just ran a regular usb3 cable through the case, outside the back panel and into the motherboard, to feed the front panel usb3 ports.
Alternatively you can a.) buy a newer 990 or similar motherboard with usb3 header, or use a usb3 addon card that has an 20pin usb3 socket like this http://www.newegg.com/Product/Product.aspx?Item=N82E16815201043&Tpk=usb%203.0%20internal%2020%20pin