The alpha release is (b)leading edge. You can run into a lot of these. Best is to use 3.0.x if you want a stable version.
Also, there are some incompatibilities between the old and the new branch. The xml elements in the gpd.xml file have undergone some changes, hence the problem. If you change the 'process-diagram' element into 'root-container' and 'transition' into 'edge' than it will probably work. A tool to provide a conversion is on the roadmap.