Why can't I paint on an existing texture? I downloaded a texture model for a tutorial and wanted to fiddle with the texture painting tools. I somehow got it to work, but after I updated blender to an newer version it stopped to work. Tried to reinstall both versions and take the clean file.

link to the blender file
http://www.blendswap.com/blends/view/74021

Help would be much appreciated
Make sure the the image editor is in paint mode. There is a dropdown box in the header, that will change the mode. By default it is set to view, so no painting.