If you are running OpUtils in a Windows Os, check whether the firewall running the OS is blocking the traps. You can either stop the firewall and check or add UDP port 162 to the exceptions list.
One possible reason could be that the MIB is not available in the MIB database. Check and Add the MIBs to the database using the Add MIB option available in the MIB Module Viewer / MIB Browser tools.
Check whether the SNMP version is correctly set in the Settings --> SNMP tab. The default version set is V2c.