Choose System > Administration > Synaptic Package Manager
On the left side of the dialog, choose the Networking (Universe) category, then look
for the package on the right called network-manager-pptp.
Click the check-box next to this package and choose "Mark for Installation".
If prompted to mark additional required changes, click the "Mark" button.
Click "Apply" on the Synaptic Package Manager dialog,
then click "Apply" again on the Summary dialog.
After the software is installed, click "Close" on the Changes applied dialog and
close the Synaptic Package Manager.