There are a few steps involved in getting SNMP functional on ESXi. They go something like this.
Set the SNMP community string
Enable the SNMP service
Add necessary firewall rules
Enable the added firewall rule
Restart the SNMP daemon
SSH into the ESX host (which will require turning on the SSH daemon on the ESX host via the vSphere client), then paste the following into the terminal window after editing the
esxcli system snmp set --communities <mycommunity> 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 /etc/init.d/snmpd restart