Glenn,
The ERD is currently a view only tool – it will only show you tables and relationships it picks up from your database, you cannot add other tables or relationships.
You are not seeing the relationships in our ERD because our tooling cannot detect any. If table A has a foreign key referencing table B, then when you select A, and look at the Foreign Keys tab in the Table / Object Info view, you should see an entry for the relationship. Ditto for B and the Exported Keys tab – do you see this?
Can you paste the DDL you used to create the two tables here? Note that in MySQL you must use the InnoDB engine for foreign keys to be correctly saved.
Hope this helps.