You need to pay the accommodation charge for CNL trains. Your pass serves as your rail ticket. You can book online at http://www.citynightline.de (select 'accommodation surcharge only, ticket exists' under 'What is required?' on the booking search page) .
You will be able to get printable online ticket.
(Here is a direct link to the English version: http://www.citynightline.de/nachtzug...en/index.shtml)

For night trains departing after 7pm, you only need to mark the following day down on your pass as a travel day.