When adding a new presentation, the Author Presenter Role is tied to Presenter Role Codes (PRESROLES), which is under Presenters in the Meetings Dictionary.
Similar to other dictionaries, you can use + Add new record along with the 
edit icon and 
delete icon to create and update the Presenter Role Codes needed. You may use the Sort Order to control how they are listed in drop downs.
NOTE: This Presenter Role Codes dictionary entry is used in the Presenter Center, and for the Presenters on an event and the related folder.