Welcome to B:bob:B!
As far as where to write the code, Designer itself is NOT a VBA host, so you will have to use something else. I use Excel mostly, but occasionally I use Desktop Intelligence as well. In the VB Editor, go to Tools, References… and add the Designer object library.
As far as sample code, I would start with this utility … Document a universe using Excel and the Designer SDK. It will at least demonstrate how to instantiate a Designer session, open a universe, iterate through all the different pieces of a universe, etc.
Dwayne Hoffpauir 
(BOB member since 2002-09-19)