SG900's ordered on or after July 1 2013 ship without the mounting brackets preinstalled.
The mounting brackets are included as part of the accessory kit that ships with the SG900.
Attaching the mounting brackets can be completed as a part of installation of the appliance into a rack.
The following picture shows the mounting bracket and its required assembly:
For further details regarding installation of the mounting brackets please see the ProxySG Quick Start Guide: SG900 Series.