on page http://www.extend-project.de/news-and-announcements/announcements/new-release-openfoam-16-ext is a link git clone git://openfoam-extend.git.sourceforge.net/gitroot/openfoam-extend/OpenFOAM®-1.6-ext
but it should be:
git clone git://openfoam-extend.git.sourceforge.net/gitroot/openfoam-extend/OpenFOAM-1.6-ext OpenFOAM-1.6-ext
without ® symbol