Hmm...
in theory, you need the forwarder dol by Narolez (USBLoaderCFG_NForwarder...you can find a link
in the first post of this thread). Put it in the root of your SD-card, start priiloader, choose "load/install file" from the menu and choose this file. Then, in the settings, change "boot to" to "installed file" (you can change "return to" to this file as well...that way, exiting a game also starts usb loader cfg).
However...up and until the beta revision of priiloader, the abovementioned forwarder simply didn't work. Others and myself used a workaround for it: we installed the forwarder from USB loader GX (you'll have to google for it, as there are multiple ones and I don't know by heart which one's the correct one for priiloader), and simply renamed configurable USB loader's path (from "SD:\USBLoader" to "SD:\usbloader-gx"). It wasn't very elegant (it prompted the intro screen of USB loader GX before booting configurable usb loader), but at least it worked.
EDIT:
this issue-handling page shows an alternative forwarder dol (
linky) which is claimed to be working. Haven't tested it myself, though.
I hope this helps.