At the moment it is not possible to book a reservation for the snalltaget night train online.
So the instructions on interrail.eu are no help at the moment.

Instead you can only book the reservation by calling the costumer service:

Tel: 0046-40-669 62 00 (landline of Malmö)

opening hours: 09-12 and 14-16 o'clock

• Have your laptop (or similar) on hand
  (to make the payment while the costumer service was still on the phone, to make sure it works)

• For the Reservation you need the full names, birthday and nationality of each passenger, your phonenumber and your emailadress

• payment was possible by creditcard or paypal.

They guided me through the reservation process very kindly! Also I waited only few minutes until I got through the waiting line. great experience!