DesktopWorkArea goes in Rainmeter.ini, not in a skin. Right-click the Rainmeter tray icon and choose "Edit Settings". When the file pops up, close Rainmeter, add DesktopWorkArea to that file, save it and then start up Rainmeter. It should work.
Note: Anything that changes the work area, such as resolution changes or resizing the taskbar, will set the work area back to its default. You need restart Rainmeter in order to regain your settings.