Open your favorite (Putty) ssh application and connect to your ESX host and then run the following commands:
esxcli system snmp set –communities public
esxcli system snmp set –enable true
esxcli network firewall ruleset set –ruleset-id snmp –allowed-all true
esxcli network firewall ruleset set –ruleset-id snmp –enabled true
The –ruleset should be a dash dash not a bar, same with the –allowed and –enabled; I don’t know why WordPress does that.