Every now and then I land up in the same issue where my destination client for restore option within Netbackup is greyed out and I have to sit and fix that before I can start my restore which is really annoying!
So here is a simple fix to that...
Go to the registry here --> HKLM -> Software ->Veritas -> Netbackup -> CurrentVersion ->Config
Under this make sure the "Client_Name" and the "Server" keys have the same value which in my case should be the FQDN of the netbackup server. (servername.domainname.com)
In my case the Client key did not show up the full FQDN of my sever so I had to edit that and once done the destination client for restores tab was no longer greyed out!