The telephoto viewfinder is slightly tricky in this way: the part that fits over your regular Diana+ viewfinder IS the telephoto viewfinder. Just fit it over the regular viewfinder, and look through where you normally would on the Diana F+.
The Telephoto Lens box was labeled wrong, which is our mistake, and is noted on the lens product page. There are not three pieces in the box, just the two you have – the viewfinder mount, and the lens itself. The part where you’d think you’d need another piece (the “accessory viewfinder”) is for use with our other Diana+ lenses/viewfinders, just so you don’t have to carry around four viewfinder mounts for the four different lenses.
So, in this way, you’re all set if you decide to get a new lens for your Diana F+!