I think the parts you are looking for are #818 - M3X18mm Male/Female Standoff. There should be 7 of these in the kit. They are packed in #860 - Huxley Electronics bag 1 (which also contains the microswitch and power jack), and is, in turn, in the box #1188 - Huxley Electronics Kit (which contains the Duet).
Email firstname.lastname@example.org if you are missing any parts from your kit, and we will send them to you under warranty. Please send your full name, address and order number. If you can send a picture of the bag that the hex standoffs should have been in, we may be able to trace why they are missing from your kit.