snmptt includes a file called snmptthandler-embedded which the ebuild does not install. This file should be installed along with the package. net-analyzer/net-snmp also must be compiled with the perl use flag which enables embedded perl on that program for this file to function properly as documented at the above url (search for snmptt-embedded).
ebuild does not install snmptthandler-embedded
ebuild should install snmptthandler-embedded