Options for configuring file compares can be found on the Compare tab in the Options dialog.
When previewing a profile, you can request to compare a source and a destination file. Before you can compare however, you need to specify the external compare tool.
If the external tool filename or path includes spaces, ensure the filename is double quoted.
"C:\Program Files (x86)\WinMerge\WinMergeU.exe" /e /x /u $(SourcePath) $(DestinationPath)