We are using SFDC Professional Edition and are running into issue when we're trying to sync custom fields from the Event object over to Marketo. Standard instructions call to log into your SFDC sync'd user and play around with end level security; however, we do not have that option with Professional edition SFDC. Marketo's CS mentoined that the object syncing now for us (EventAttendee) is actually a child object to the parent Event object.
As further context, when our sales reps finish meetings, they're updating a pick list custom "results" field. The event object seems to already be syncing but it's limited with it's options and we can't seem to pull in any other custom fields.