Tested it again, and it does work for me. There are two search bars on the page. One at the top, that is a general search of the Tropenmuseum website, and one in the center frame. It is the one in the center frame that you need to use, as it searches the collection. It says 'enter your search term'
Have you tried copy-paste with the code? Perhaps you are using a different symbol?

Anybody else having any difficulties?