Use the vicfg-snmp command from Perl SDK:

# send traps to centreon.example.com at standard udp port 162 and community public
vicfg-snmp --server esxi.example.com --portnumber 443 --username root --password rootpass \
 -t centreon.example.com@162/public

# start the snmp service:
vicfg-snmp --server esxi.example.com --portnumber 443 --username root --password rootpass --enable

Test with:

vicfg-snmp --server esxi.example.com --portnumber 443 --username root --password rootpass --test

You should see something like these in /var/log/snmptt.log

Wed Jan  4 10:47:27 2012 .1.3.6.1.6.3.1.1.5.2 0 "Status Event" 192.168.1.253 - SNMP is reinitializing

A copy of the MIB

Extract the mibs to /usr/share/snmp/esx.

Add mibs to snmp system, edit /etc/snmp/snmp.conf :

mibdirs +/usr/share/snmp/esx