The bios in all my rigs will only show a usb boot option if I have a usb device plugged in. I would bet yours is the same way plug a usb bootable stick in and it should show up as a option.
That makes sense now that you mention it, although in my limited experience (very!) in other BIOSes there was an option in the BIOS to boot from a USB device.
Any way to access boot options on start up? As I said the splash screen is only on for a second or less so I can't tell if it's on there or not.