You want to repeat every 10th day, excluding weekends? So the pattern of periods on Monday October 15th will next occur on Friday October 26th? That is not possible with iCal/Calendar, as far as I know.
The problem is with the specification of the calendar standard - first it applies inclusion (every 10th day) and then exclusion (not weekends). If you edit the event (outside of calendar) to
it still counts the weekends, but drops the event if it falls on one.