From VMware vSphere CLI, run:
vicfg-snmp.pl --server ip.of.esxi.host --username root --password EsxiPassword -c public vicfg-snmp.pl --server ip.of.esxi.host --username root --password EsxiPassword -p 161 vicfg-snmp.pl --server ip.of.esxi.host --username root --password EsxiPassword --enable