It seems difficult to get a ticket at the normal price of 12 €. (A waiting line they not move forward.)
For 30 €, there are VIP tickets available, which must be booked in advance or at Berlin Tourist Infos.
http://www.imexinberlin.de
( Just for info )
Is it open to people with the 3-day Berlin Museum Pass as well, or only with a special ticket?
The exhibition Impressionismus – Expressionismus is not included in the 3-day Berlin Museumspass. Time slot tickets are available at Berlin Tourist Infos. (VIP tickets for 30 €)