Hey Carolin. Thanks for the interest. It will work with Thesis, but admittedly that is one of the more problematic themes we’ve encountered. If you take a look through the existing Thesis-related discussions (https://theeventscalendar.com/?s=Thesis) you’ll see that some users have had problems integrating.
We’ll do what we can to assist users who are facing problems, and there is a tutorial Matt Hodder put together a while ago about integrating the two (http://www.matthodder.com/events-calendar-pro-for-thesis/) that we’re going to be updating for 2.0 shortly. But I’d try the free The Events Calendar (http://wordpress.org/extend/plugins/the-events-calendar/) first to see how it integrates before buying PRO. Since they’re the same code-base, if free works you’ll know PRO should too.
Hope that helps! Let me know if not and I can elaborate.