The 2015 NEM summit edition will be held at the Frankfurt Book Fair (Frankfurter Buchmesse, Ludwig-Erhard-Anlage 1, 60327 Frankfurt am Main, Deutschland).
Frankfurt Book Fair 2014
Accommodation in Frankfurt can be found here: http://www.frankfurt-tourismus.de/en/Accommodation.
Private rooms are a quite good alternative. As the city gets crowded and quickly booked for the Frankfurt Book Fair it is highly advisable to make arrangements well in advance.