Logging for the Desktop Client is different than logging for any of the Service components. The registry key is in HKEY_CURRENT_USER, not HKEY_LOCAL_MACHINE.
To the key HKEY_CURRENT_USERSoftwareOSSIIntelli-SiteDesktopClient add the DWORD value LogToFile. When this DWORD is set to 1, logging will occur. You may need to exit the Desktop Client and launch it again to have it take effect.
Note: The Registry Key will not appear until the setup dialog in the Desktop Client has been run. The setup dialog is the gear on the login dialog. Click it, then click OK.
The log file, DesktopClientLog.txt, is written to the "My Documents" folder.
Note: For any of the OEM brands, replace OSSI with the company name and Intelli-Site with the brand name.