When a Related Item uses a Poly Item as its source, a user will see an additional dialog when creating a new Related Item.  When creating a new Related, you must first select which of the Poly Source types you wish to create. For example, if you are creating a new Deliverable for a specific line item in Project, here is the first dialog that would display:

Select the Type of Item, and the rest of the process of Item creation should be familiar.