The hotfix is needed for the appliance to know the command "upgrade-appliance"
if you are unable to get the HF I would open a case at symantec.
some considerations to take in count:
use the remote powershell not from windows 8 or windows2012, this powershell version is to recent for the appliance.
the following command needs also some special attention:
enter-pssession -computername <MUST be the FQDN> (ip-address will not work) -credentials <must be the 'appliance administrator'> (backupexec service account or domain admin will not work) -configuration appliance
if you launch this command from a supported powershell version and the HF is installed and you run the upgrade with the installation on local disk or USB it should work.