Debian 6 doesn't have GTK+ version 3, so you need at least version 7, codename Wheezy. Siril now uses a GTK+ 3.6 while Wheezy only has the 3.4. For a desktop system, the next stable is probably the better choice, called Debian testing, currently version 8 with codename Jessie. We maintain an official debian package which is in the debian unstable repository. It will also be available in Jessie and on other debian-based systems soon. Until then, or if you want to build the latest version from source, use the procedure below.