It's really easy. Just hold your ticket so it matches the picture on the machine. Put in the slot that says "Insert Park Ticket Here." The bar code faces up and should be on the left side.
If you've inserted the ticket properly, the FASTPASS will come out of the slot labeled "Receive your FASTPASS here." Check to be sure it's a valid FASTPASS. If you're holding another FASTPASS and have arrived a few minutes before its start time, you can't get another. If that happens, just check the times and come back in a few minutes.