Hi Clara
I would agree with Rob...create your BO/Class Default method to call that BDC Screen.
If you are using a BO, set that method as Default Method in the BO Details.
If you are using classes, call the BDC from EXECUTE_DEFAULT method of the IF_WORKFLOW Interface used in the class.
Remember to pass an instance of the class/BO from the Workflow to the Task
Users will get a link in the decision step (under "Objects and Attachments").
The Text of the link will be the default attribute and the BO/Class Short text. Clicking the link will call the default method.
regards,
Modak