look at the connector that goes to the board. If its a little 8-9 pin black one then its USB 1-2. If its a bigger blue connecter then its usb3. You can not plug a usb3 case header into a usb2 motherboard header... it wont fit.
i looked at all the cables, and i found a thick black one with a clearly labelled "USB 3.0" on it, and a seperate little one for the 2 USB 2.0's. this is actally a little annoying, since they shouldn't actually be there. i'm putting my new build into it today, unfortunately i don't have a USB 3.0 header in my motherboard