I am doing an add on directly from Disney. I know that the 2010 dues will be prorated. Are the dues prorated from the time I initiated the add-on, or from the date the add on closes?
They will be prorated from the time you begin the process - not when the transaction closes unless it's a resort that has not yet opened. Since you can make reservations immediately, the dues start immediately too.
The dues begin on the later of: your purchase date, the start of your initial Use Year and the occupancy date of the Unit you purchase. The occupancy date only applies if you are purchasing a resort that is still under construction and since all of the resorts currently being sold are fully open it would not apply. DVC normally starts your contract with the current UY so the start of your initial UY is normally already past in most cases, again unless you are purchasing new construction. So that leaves the purchase date as the "later of" the three dates in most cases.
Disney pro-rates based on the number of days left in the year so if you purchase today (Mar 29) you would owe 277 days worth of dues for 2010.