You have the black plastic rails yes?
Ok get your optical drive, get one of the rails.
the metal tab needs to be pointing to the front of the drive, the recessed bit also needs to be on the outside, as in the flat side of the rail flush against the side of the optical drive.
Then simply fix the drive rail to the optical drive using the screws, be aware when attaching the drive, make sure that the metal tab, the very end is inline with the front of the drive.
Repeat this on the other side.
You should have your drive with the rails fitted and the recessed part facing outwards and the metal tab of the drive rail pointing towards the front (disc insertion side) of the drive. And finally the metal tab should be pretty much inline with the front fascia of the DVD drive also.
You can now insert the drive into the front of the case (after removing the blanking plates)