We will be using Werner Jaeger l2tp-ipsec-vpn software.
First we install l2tp-ipsec-vpn from a terminal window and reboot the computer so all changes are applied:
sudo apt-add-repository ppa:werner-jaeger/ppa-werner-vpn
sudo apt-get update
sudo apt-get install l2tp-ipsec-vpn
gsettings set com.canonical.Unity.Panel systray-whitelist "['all']"
sudo shutdown -r now
The gsettings line is to allow the systray L2TP/IPSec configuration/connection icon to appear on System Tray (just necessary for default Ubuntu Natty Narwhal desktop manager, Unity). More info here.
Now we should have a new icon in System Tray area showing two computers. This is the L2tp IPSec VPN icon.
We right click on it, choose Edit Connections.
We add a new connection named i.e. TUVPN_L2TP_IPSec.
We configure first Tab IPSec as follows:

L2TP/IPSec IPSec Tab
So we just need to type the desired VPN server we want to connect to and the pre-shared key where we type 'tuvpn123' without the quotes.
The L2TP Tab we leave it as it is, no change.
Finally on the PPP Tab we click on Allow these protocols and we mark Microsoft CHAP Version 2. We also enter our VPN username and password:

L2TP/IPSec PPP Tab
We don't change any other setting. We are ready to go!
So now we go back to the L2tp IPSec VPN icon, we choose the newly created L2TP connexion and that's it:

Connect L2TP/IPSec
Confirmation of successful connection
If you wish to confirm that you really are now enjoying new freedoms, security and anonymity on the Internet, go to an IP identification site (e.g. www.whatismyipaddress.com ) where you can see that your IP is now a TUVPN server node in an entirely new location....and probably a new country!
Enjoy your new freedom and security!

Confirmation of successful connection
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