If it is thishttp://www.asrock.com/mb/overview.asp?Model=Z68%20Extreme3%20Gen3
It does not seem to have the onboard 20-pin connector.
That means you will not be able to use that adapter. There are other Asrock board with the 20 pin usb 2.0 connector, but not that one.
Not ALL cases work with that connector you linked, since your case needs to have a matching spot to screw those in.