$ sudo apt-get update
...
W: GPG error: http://packages.freecontrib.org dapper Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F120156012B83718
W: You may want to run apt-get update to correct these problems
$ sudo chown -R ${USER} 
$ gpg --keyserver subkeys.pgp.net --recv F120156012B83718 
gpg: requesting key 12B83718 from hkp server subkeys.pgp.net
gpg: key 12B83718: public key "Lionel Le Folgoc (mr_pouit) <lionel.lefolgoc@free.fr>" imported
gpg: Total number processed: 1
gpg:               imported: 1
$ gpg --export --armor F120156012B83718 | sudo apt-key add -
OK
$ sudo apt-get update

$ http://easyubuntu.freecontrib.org

-------------
$ sudo apt-get install build-essential
$ sudo dpkg --install file.deb
$ sudo apt-get -f install
$ sudo apt-get autoremove