If you are trying to open any of the pre-designed templates and the program is closing, then I think your installation is corrupted. A complete uninstall, then a clean re-install will fix that.
I am having the same problem as Tobiann. I select "open an existing project" and the application closes. I have uninstalled and re-installed the program. I have installed the most recent update, I believe is 3.5.2. I am still having the same problem. What can I do to fix it?