Connect Unmanaged Switch to Fortigate 140D-POE via SFP (DMZ) Fiber Module

First off, let me say that I am a FortiOS newbie and therefore do not assume I know anything.  I have managed to get the Fortigate mostly configured the way we want it, but I am completely stuck on this one point.  The Cookbook has done me no good, and neither has Fortinet Support.

We formerly had a configuration which used consumer-grade equipment and looked like this:

Cable Modem <--> Router <--> Cisco Small Business Switch <-Fiber-> Cheap Unmanaged Switch on Campus edge

With the installation of the Fortigate and not having the fiber port working, we have:

Cable Modem <--> Fortigate <-Network Cable-> Cisco SB Switch <-Fiber-> Unmanaged Switch

What I want to do is to move the Cisco switch to replace the unmanaged switch and use the SFP/DMZ1 port on the Fortigate to talk to the Cisco switch via multimode fiber.  I have a FG-TRAN-SX SFP module in DMZ1 port on the Fortigate.  The diagram should be:

Cable Modem <--> Fortigate <-Fiber-> Cisco Small Business Switch

I need to know how to get the DMZ1 port communicating with the Cisco Switch so that the ports on the Cisco are on the same sub-net as the network ports on the Fortigate.

I hope I have been clear, but I have no idea if I have been.  Please bear with me.  I need help.