@klaasvaak. Thanks for posting. This is clearly answered in the FAQ. Please be advised that although Easy Folder Listing Pro 2.8.5 and lower are compatible with Joomla 1.5, as of the next release, it will only be compatible with Joomla 2.5+.
Also, unless I can look at the site, it can’t say exactly what the problem is. There could be other factors as well, such as presence of jQuery. However, most likely, your Mootools version on the site is too old. You need to enable the the Mootools upgrade. Also, you need to have the last released version of Joomla 1.5.x.