"A stage design does not really belong or desire to be in a museum," writes German designer Johannes Schutz, who made this sketch of "Poppea" in preparation for a 1993 Dusseldorf production of Monteverdi's The Coronation of Poppea. "Its proper destination is a theater." Nonetheless, designs for sets by contemporary German and Austrian designers Schutz, Achim Freyer, Dieter Hacker, and Erich Wonder will be on display in "Stage Design Today--Stage Design of the Future." The show opens Thursday, April 28, in suite 842 of the Merchandise Mart, Wells Street and Kinzie, with a free reception from 5 to 7. Regular hours are noon to 6 Tuesday through Saturday. Call 329-0915 or 443-4751.