I've downloaded these DEB packages from http://sourceforge.net/projects/sstp-client/
- libsstp-client0_1.0.9_amd64.deb
- sstp-client_1.0.9_amd64.deb
- network-manager-sstp_0.9.8.8-0ubuntu1_amd64.deb
- network-manager-sstp-gnome_0.9.8.8-0ubuntu1_amd64.deb
first and second are installed successfully, but 3. step is critical for me. Because network-manager-sstp_0.9.8.8-0ubuntu1_amd64.deb needs package ppp version 3.4.6 or lower and in my system is version 2.4.6-3.1ubuntu1 ... please help
If anyone else still has this problem, just use the PPA sources mentioned on the page above. I.e.,
This will install network-manager-sstp 0.9.10.0, for which the ppp version is not a problem. This fixed it for me.