Not sure if loading EFI shell from a USB drive is supported. I run the EFI shll from the system drive's ESP.
You need to mount the ESP and place shellx64.efi at the root of the partition. Then you'll be able to boot it.
To mount the ESP do this:
diskpart
sel disk 0
list par
find the ESP partition number (in default win partitioning scheme it should be partition 2)
sel par 2
assign letter N
At that point it should be mounted as N. By default Win8 won't let you open the partition for viewing under Windows Explorer. So copy shellx64.efi to the root of N via the admin command prompt.
🙂