The first step is temporarily enabling the ESXi shell:

The next steps are all issued from the ESXi CLI. Basically enable snmpd to listen on port 161, define the community, and enable it.

esxcli system snmp set --port 161
esxcli system snmp set --communities public
esxcli system snmp set --syslocation ""
esxcli system snmp set --syscontact ""
esxcli system snmp set --enable true

After this is configured, the ESXi host will respond to SNMP v2c queries using the public community.