FortiClient not recognizing Fortigate for Mgmt

We're looking at switching to FortiClient for our antivirus and endpoint protection solution, and I'm having trouble getting it to work with our Fortigate box.

We have a 90D running FW 5.0.9, and I downloaded FC 5.4 from the licensing tab in the GUI.  It downloaded and installed fine, but when I tried to register it to the 90D for management, is says "EMS or FortiGate not found".  Is there something I need to set in the Firewall to recognize it?  I saw Endpoint Control, but that forces FC use and we often have guests and customers come in that we want to allow web access.

Any ideas are appreciated

Thanks