In this post I am assuming that you are running ubuntu desktop with gnome.
The first step is to install the appropriate package:
sudo apt-get install network-manager-gnome network-manager-pptp
Next, click on the network manager icon
VPN Connections -> Configure VPN...
Then click Add
After that enter your gateway, username, and password.
Then click Advanced...
The settings you want to change here are, uncheck EAP (in top list) and check MPPE
Click Ok then Apply and you are done