Not officially supported. You can request root password by opening a support ticket, and install/enable net-snmp. It is not guaranteed to persist with version upgrades.
HolyGuacamole has provided one solution. There is another...
If you are running a trial, and want to see what you can get out of SNMP-based monitoring, you could read up on how to reset the CentOS root admin password, as that is what the appliance is built on. I won't say any more than that, but please note that this option is NOT SUPPORTED WHATSOEVER AND SHOULD NOT BE ATTEMPTED ON A LIVE, LICENSED VMAN APPLIANCE! /disclaimer