SNMPTT with snmptrapd is a powerful tool to integrate traps within Nagios.
What you need to remember is, that snmptt is waiting for its configuration file (snmptt.ini) in /etc/ by default.
The normal procedure to use snmptrapd with snmptt is to use a traphandle within /etc/snmptrapd.conf:
traphandle default /usr/sbin/snmptt --ini=/etc/snmptt/snmptt.ini
If you have your snmptt.ini somewhere else than /etc/, use the format above.
snmptrapd will then forward all its messages to snmptt, which will do whatever you want with the traps.