Sorry I was not clear - if the questions are specific to pi4soa tools then best to use the pi4soa forums. However, if related to the use of CDL in governance, then here is fine.
From your description of the problem, I think it is probable that you don't have the link established between the scenario and the choreography. If you click on the background of the scenario, the properties window should show a 'Choreography Description URL' property - if you enter the relative path to the .cdm file, you should then see the list of participant types defined in that choreography appear in the participant type list.
Once the link to the choreography is established, you will also be able to select the defined messages when creating the links. Without the choreography URL defined, you would need to manually enter the information (e.g. message type, optional operation, etc).
If that is not the problem, then you may need to send me the choreography and scenario.