1. Required packages
You’re gonna need network-manager-pptp package installed.
If you can connect to the Internet somewhere else (like in a Coffee
bar or sth) open Synaptic package manager, search for and install the
network-manager-pptp package (or enter sudo apt-get install
network-manager-pptp from a terminal).
If you don’t have Internet available anywhere you can also find
this package on the Ubuntu install CD if my memory is good (somewhere
buried in the folders).