Are you in SQL Developer or SQL Developer Data Modeler?
What version of the tool?
Is this a model you created or someone gave you?
What do you see in the tree after you open, is the design shown and you just don't see the diagram?
In other words, we need more information to help you.
Do you have the Data Modeller Browser or Navigator visible? (View | Data Modeller | ...)
Is Oracle Designer (2000) still available ? I rather liked old O2K ; OK , it had its moments , it wasn't perfect , but it sure left this heap standing at the lights . . .
You may well be better off with another product, from your posts here, you plainly don't understand SQL Developer and don't seem particularly interested in letting people help you understand it.
Why would we want to help you after you insult our hard work?
I'm guessing you haven't opened the model browser. Once you have the browser open, you can right click on your relational model and say, 'show' - this will open the diagram. If this doesn't work, your SQL Developer desktop layout might be screwed up and needing a reset.
View .. Data Modeler .. Browser