Install the RCLI on a pc/laptop/server. Got to the command prompt and enable SNMP.

vicfg-snmp.pl --server <your_host> -t snmptrapsystem@port/community -c communitystring

(@port is optional in this command) - you can also set the community with the -c switch

vicfg-snmp.pl --server <your_host> -E = enable the service
vicfg-snmp.pl --server <your_host> -E = stop the service

vicfg-snmp.pl --server <your_host> -T = test your setup
vicfg-snmp.pl --server <your_host> -s = show your setup
vicfg-snmp.pl --server <your_host> -r = reset your config

(tested on a 4.1 ESXi host)