You can open a Diagram Window (File->New->Diagram Window), select all tables in the Object Browser (open the "Tables" folder, click the first table and shift-click the last table), and drag all tables into the Diagram Window.
Then you can spend the next 2 hours dragging windows and lines around to make it look pretty, and then, hopefully, you'll have what you're looking for.
(If there's a better way to do this, either with PL/SQL Developer or another tool, I'd love to hear about it too.)