On current documentation page: http://docs.mongodb.org/manual/administration/maintenance/
there are 2 mentions of SNMP Trap:

Monitor MongoDB With SNMP on Linux
The SNMP extension, available in MongoDB Enterprise, allows MongoDB to report data into SNMP traps.
Monitor MongoDB Windows with SNMP
The SNMP extension, available in the Windows build of MongoDB Enterprise, allows MongoDB to report data into SNMP traps.

However it appears that you cannot make mongod to send SNMP trap in anyway. There isn't even a place to configure trap destination.