Backing virtual FreeBSD based appliance

According to the BE2012 software compatibility list, FreeBSD is not supported (not listed):
  http://www.symantec.com/business/support/index?page=content&id=TECH175581

So that means installing a BE agent the traditional way to backup files from this VM is out of the question.

But since we are dealing with a VM here, some might think that it may be possible to tell BE to do a snapshot of the VM and back that up whole, i.e., via the vStorage API backup methods.
(Some refer to this as the Backup Exec Agent for VMware Virtual Infrastructure (AVVI) method, but I think the name AVVI is now depreciated.)

One of the steps involved in a vStorage-method VM backup is that the VMware Tools inside the VM will be contacted to do some snapshot preparation work.
(And if the BE agent is installed, it will be contacted too, but this step also requires the BE server to have root access credentials to the VM, which you do not have.)

Since your VM has neither VMware Tools nor the BE agent installed, the vStorage-method backup will not be successful.

It might be possible to get the virtual appliance support vendor to help with installing VMware Tools for you, but since BE2012 doesn't support FreeBSD, you still cannot install the BE agent.

To rephrase everything: You probably cannot backup this virtual appliance using Backup Exec. You may want to ask the appliance vendor for their specific HA and/or backup solution.