www.dd-wrt.com
to enable usb in dd-wrt or in the default firmware?
In the dd-wrt i compiled the modules for it so if u need it i can send them to you
For the default firmware, make sure the usb disk /stick has standard partions (i had 1 with 4 partitions and didn't recognised my stick , so i had to make a single partion and was working ok).