To enter externalized services personnel, click on PERSONNEL in External Services.

Name, Function, Type of Service.
Type of Service contains all services specified in Service List window. If you didn't specify any services there you can't enter service provider personnel.

If a person works as a 'Centralizer', i.e., 'Type of Service' is "Central Laboratory" or "Core Laboratory" and he/she is processing your centralized procedures (e.g., ECG, lab samples, etc.), you have to add him/her to the Study/Country Associated Personnel List to be able to select him/her as 'Centralizer' for the respective procedures listed in the Centralized Procedures window.