When purchasing online through their website, you can only choose a seat if the flight is within 90 days of booking. You have to click on the seat icon before proceeding to payment. You cannot go back after payment has been processed to choose a seat!
Unfortunately if buying/booking more than 90 days, you cannot choose a seat at all through their website.
However you have the possibility of phoning their callcentre 7 days before the flight to book a seat. That is the earliest they will allow to reserve a seat.
Otherwise you also have the possibility of choosing a seat during the online check-in 24hrs before the flight (note: some airports where they fly from do not have this option!)
Hope this helps!