It's not obvious, but set your instance of IEdmEnumeratorVariable5 directly to your instance of IEdmFolder5. Like this...
' -- assuming you already have a folder instance named eFolder --
Dim eVar As IEdmEnueratorVariable5
eVar = eFolder
eVar.SetVar("Description", "", "THIS IS MY FOLDER DESCRIPTION", True)
eVar.Close(True) 'close the folder and save updates