It took me quite some research and contacting support to find this out ... This is due to the fact that the sourceforge repo only contains the tools, not the model itself (which is not very obvious).
That's why one should first install it via the official installer: http://www.cio.bund.de/DE/Architekturen-und-Standards/V-Modell-XT/vmodell_xt_node.html and then maybe update the tools via sourceforge binaries.