The file DiffMergeCtl.ocx need to be registered with VSS application 
in order to provide VEB with such services as 
"File View"
"Show Files difference"
"File Merge"
for files kept on Sourcesafe database.
In order to register DiffMergeCtl.ocx file with your local VSS:
1. Exit VEB
2. Copy the file into VSS\Win32 folder on your local drive wherever VSS is installed to.
3. Execute "REGSVR32 DIFFMERGECTL.OCX" command from that place.