Glad to help Steve
First, the rails you are looking for are not rails after all (that's what threw me off too!), but rather are 4 screws with large rounded heads. They are (or should be) screwed into a small piece of plastic mounted vertically just to the left of the drive bay opening. Screw them into the mounting holes on the side of your new drive. Then simply slide the drive into the bay; the screw heads slide into the plastic slides. Plug in the connectors, pivot the little holder (on the right side) down to keep the drive in place, and close up your case.
What I did was to simply boot the machine. During start up a message came that said the machine did not recognize the drive, and Disk Utility started automatically. I highlighted the new drive, assigned a new name (mine is Macintosh Drive B) and clicked the erase button. Disk Utility took care of the rest. Conversely, you may choose to partitian the drive instead. Click that tab, and create the partians as you choose. I did not use the Seagate utilities.
Good luck, and enjoy your Seagate drive. I have had wonderful success with them.
Blessings,
Bob