http://www.interrailnet.com/planning/train-seat-reservations/how-to-make
You can make your reservations for most European trains through railway call centres. Reservations can be made even before you've received your InterRail pass. Make sure you mention that you have an InterRail pass and that you only want to make a reservation.
We suggest you make your reservations through one of the following call centres:
Deutsche Bahn booking centre
Phone number: +49 1805 996 633 (open 24/7)
When calling, you'll hear a menu in German. Select extension "9" for an English menu, then "1" for train information and bookings. The DB booking centre wonÂ’t charge you any booking costs. The delivery cost is EURO3.50 for every country.
You can also pick up a confirmation of your reservation in larger German train stations. Just bring the order number you received on the phone to a 'self-service' machine or a ticket office in the station.