In Windows Explorer, if I navigate to "Desktop," I can see my "Dropbox" system folder.
If I do the same in DirectoryOpus, the folder is not shown in power view, details view, or icon view. The other special system folders like "Skydrive" do show up correctly. However, in the folder tree, the Dropbox folder is shown.
If you go to Preferences / Folders / Virtual Folders you'll find options for various things to be shown in the Desktop folder.
I'm not sure if Dropbox will be one of them, but if it isn't then you can turn off Native display of the Desktop, including the following in the same place, and Opus will then delegate display of the folder to the Windows shell, giving you exactly what you see in Windows Explorer (but with reduced functionality while in that folder).
Alternatively, you could just create a shortcut to Dropbox in your Desktop folder. (If you do that, you might want to turn on Preferences / Folders / Folder Behaviour / Sort shortcuts to folders like folders, and also hide the Dropbox item in Explorer so you don't have two identical icons, which you can usually do by right-clicking it.)
Another consideration, which many people prefer, is to create toolbar buttons or hotkeys, or menus of common folders (sometimes in addition to the normal favorites menu) for commonly used folders, which often let you get to them much quicker than having to go via Desktop, and reduces clutter from the desktop folder.