You have a lot of templates available here to play with your appliance.
Still didnt help, Get the appropriate OID for your interface and you can monitor it thru MRTG. You may also use a SNMP Walker to find the correct OID for the virtual interface. Put that cryptic number in your config and you are through
BTW, though its related to a Firewall appliance, I dont think this post should be in this section.