I want to generate an ER diagram for an existing database. I understand that there is some tool in the Developer/Designer Suite that allows one to do this. Or, it may be possible to gernerate an ER diagrm by some other means. Any thoughts??

You can reverse engineer an existing schema using Oracle Designer.
Use the "Capture Design" functionality. The document I have attached gives you a step-by-step for Oracle Designer 6i that I created for a client. Let me know if you have any other questions.

Thank-you