The dues are pro-rated from the later of your purchase date, the start of your initial Use Year and the occupancy date of the unit you purchased. If you purchased a unit that is already open and your initial UY is Oct 2010, you will owe dues starting from your purchase date. They will send you a bill after you close on your add-on. If you are paying by monthly debit, the first payment would be deducted starting the month following your closing date, April 1st or 15th in your case. Your monthly payments will be higher the first year because you will owe N months worth of dues, paid in N-1 or N-2 installments.
If you purchased Aulani and your first points are Oct 2011 points then your pro-rated dues would start Oct 1st and deductions would start on Nov 1st/15th.