Hi Geoff,
You don't need any MIB. Regarding OC 5.1 trap listener, is there any other application holding port 162? can you check OnCommand Events Page for warnings. If port 162 is bound to some other application DFM can't receive any SNMP traps.
In parallel, can you try with 3rd party trap listener (Eg., http://www.manageengine.com/products/mibbrowser-free-tool/download.html).
Enable all options under 'SNMP' tab (can be found in Tools->Wfa configuration). Execute a simple workflow.
thanks
Giri