Open the side panel, look at the top of the optical drive bay, you will find 4 button type screws and engraved instruction for installation.  Attaching 2 screws to each side of the blu ray drive and it will serve as drive rails for gliding into the drive bay. Edited to add visual instruction video.
I recently bought a Dell Precision 5820.  I also bought a 7MM6C BluRay  drive to install in to it.  My problem is that there seems to be a need for installation railings for the drive.  I can't find those optical drive install brackets anywhere!  They unfortunately were not included with the 7MM6C.  I've put in a chat to Dell sales, and they have yet to get back to me.  Can anyone give me the part numbers I can search for those brackets somewhere or can Dell find them in an inventory somewhere to either sell them to me or send them to me? 
Dell Precision 5820 - installation of 7mm6C Blu Ray -- need installation 
brackets