I don't know how to make it work, but there is a Show Desktop docklet available.
With OD+, all you have to do is right click on an empty area of the dock, go to Add, and select the "Show desktop docklet".
I'm not sure if this available in the free version, if not, just do a search for the docklet.
There's something you need to know about Show Desktop... using it can hide your docks, and widgets (if you have any), along with your windows. Here's why...
"Show Desktop" does not minimize anything. What really happens is a full screen, borderless, explorer window, opens on top of everything.
You can use.
Winkey + M to minimize all
Winkey + shift + M to restore all
Or
Copy and paste the following into a text file and save it as ShowDesktop.vbs. Then create a new shortcut linked to this file.
Set Shell = CreateObject ("Shell.Application")
Shell.MinimizeAll
Set Shell = Nothing
and
Copy and paste the following into a text file and save it as RestoreAll.vbs. Then create a new shortcut linked to this file.
Set Shell = CreateObject ("Shell.Application")
Shell.UndoMinimizeAll
Set Shell = Nothing
Now you have more than one option.