It would be really nice if my Dropbox directory structure was linked to a top level item in the folder tree - eg. under "File Collections"
When saving a file - I can find my drop box under "Favorites" in the save file dialog box. To open my dropbox in Directory Opus the simplest way is to double click the dropbox icon - this is not very elegant
You can add toolbar buttons or hotkeys as quick ways to get to folders which you use often.
Aliases are another option (e.g. you could type /d to jump to the dropbox folder).
I use both, with toolbar buttons to go to things I use every day, and aliases to go to things I use less often. (And some things I use even less often are in the normal Favorites menu.)
If you want to use the folder tree, and ensure there's an item for Dropbox near the top, you could create a Library containing just your Dropbox folder, which would then appear in the Libraries branch at the top of the tree.