How to make a relationship between two Business Data Model? Please help

In your requerimentos business object don't use the type Integerfor CIUDAD attribute but directly select ciduades in the type drop down list. You can then choose between Aggregation and Composition. A unique id (called persistence id) is automatically generated for each business object you create.

A quick note: you actually create a relationship between two Business Objects not between two Business Data Model: Bonita BPM currently support the definition of a single business data model.