Hello,
the full RPL integration is not available for activities. As mentioned you can call the function WFDS_SEARCH_RESOURCES. You need to provide the correct parameters for finding an employee suitable for the activity. This however is used for performing the search. There is no real assignment created.
The call needs to be done from BAdI CRM_MKT_MODIFY_ORDER, you can then use the employee returned from WFDS_SEARCH_RESOURCES as employee responsible for the activity created from the campaign execution. There is however no full integration available for activities.
regards,
Johannes