scheduling in plant maintenance &external scheduling

hi expert ,

can you  explain the scheduling in plant maintenance step by step beside external scheduling and how to made the external scheduling in IDS system also i read thesap help links about external scheduling  and add the external scheduling in the order type pm01 and add the:

Function Module PM_ORDER_EXTERN_SCHED_DIALOG in Configure Communication in the (D: For scheduling in dialog with external scheduling) should bedialog box Scheduling agreement appears after the order has been scheduled and this is not appear and information message  (Link to the scheduling system could not be built) is appear.

is any configuration missing.

thanks sadin