This motherboard treats USB drives as another internal hard drive plugged into your MB. The USB drive is in the "hard drive" options. So, use F12. Press "Hard drive" and NOT the USB drive. Then it'll ask which drive you want - choose your USB drive there.
This is also a great way to multi-boot with Win7 without needing to partition your hard drive. Simply install Win7 on another drive - I unplugged my Snow Leopard drive first. Then plug Snow Leopard back in after Win7 is installed. Use F12 whenever you want to boot to the Win7 drive - again, select "Hard Drive" on the first screen, then select your other drive. Oh, Chameleon updates have fixed the reason I used F12 for Win7, so that's not needed anymore. But it's still really easy to do!