Look in this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\SourceCodeControlProvider\InstalledSCCProviders
The value for TFS is the HKEY_LOCAL_MACHINE location for provider specific settings. PB Native would be Software\Sybase\PowerBuilder\12.5\Native SCC. Under the TFS provider specific key you should find the following:
SCCServerName The name seen in PowerBuilder's SCC property tab
SCCServerPath The full path and file name of the SCC API interface library. For PB Native it is C:\Program Files (x86)\Sybase\Shared\PowerBuilder\PBNAT125.DLL