NetBackup Media Server cannot be clustered in 7.x - both nodes are installed as independent media servers, but clustered 6.5 media server can be upgraded to 7.x in its clustered form.
Can you confirm that NBU is indeed clustered? In its own resource group with own Virtual hostname?
The way we do it in 7.x is to install media server on each node and then use nbemmcmd commands to create 'app-cluster' using the same virtual hostname as the application (e.g. Exchange) virtual name. STU is then also created using virtual hostname.
NBU upgrade steps are listed in NBU Installation Guide.
Steps for clustered master and/or media servers are listed under this topic in Installation Guide: Requirements for Windows cluster installations and upgrades
You need to upgrade to 7.1 first, then patch to 126.96.36.199.
Upgrade portal: http://www.symantec.com/docs/TECH74584