What about the reverse? I need to go from an Erwin model into Designer. If that's not possible, is there any way to reverse engineer an ERD from an existing database or DDL file. I find Designer to be not at all intuitive. I need help.
Yes, that's possible as I just did it myself (in Designer v6). I have never used Erwin, but you will somehow need to generate the DDL from your physical model in Erwin and specify Oracle as the target database before you can reverse engineer. I assume this is fairly straightforward in Erwin.
Assuming you have this DDL SQL file (I was sent mine), open the Design Editor in Designer, and go to Generate -> Capture Design of, and choose 'Server Model' from the list. In the Source of Design Capture, select 'DDL Files' and locate the folder and the SQL file in Windows Explorer. Then select your version of Oracle and click on Start. (There are some other options if you want to be specific.)
A Server Model Diagram ('SMD' = Physical Model) will be produced with the object definitions specified in the DDL SQL, e.g. tables, triggers, views, keys... Btw, before all this, you will need to have an Application System open in Designer which will hold the Server Model.
The only problem you have is that the SMD is a Physical Model, not an ER Diagram (Logical), so it will have tables and not entities. There is a Table to Entity Retrofit option in Designer, but this will create an ERD which will exactly match your SMD - from a business perspective, this may not be correct (i.e. some entitities don't become tables, such as system or processing tables) and is something to bear in mind.
Another option, if you have an Erwin-specific file (with a .er1 extension, I think) is to use a third-party product to convert this. Have a look at http://www.metaintegration.net/. for more info on this, as they are a vendor which can do this.