Ticket sales for the Delft Chamber Music Festival are handled by the Nationale Theaterkassa. 
Most of the concerts in the Van der Mandelezaal your seats are marked. For the other concerts free choice of seats applies. The tickets will be sent by e-mail and must be printed out to the concert, along with your discount card (Delft / Rotterdampas, UITpas or CJP, student card).

Here's the manual to order tickets.

Here's the link to the complete order overview.