The T in T3600 is for 'Tower'. The optical drive is vertical by default when the unit is in it's regular vertical position.
Pretty much every picture Dell has of a T3600 shows it in tower (vertical) format, and the online setup guide shows it in vertical position.
The vertical positioning should allow the optical drive to work just fine.
I'd suggest to check in the bios if the sata port is detecting the drive (or maybe the sata port is disabled). If all the sata ports are turned on, but it still will not see the optical drive, you may want to reseat/check the power and sata cables to the drive.
