Habilite os repositórios Universe e Multiverse em seu sources.list
sudo gedit /etc/apt/sources.list
Edite isto:
deb http://us.archive.ubuntu.com/ubuntu/ precise main restricted
deb http://us.security.ubuntu.com/ubuntu/ precise main restricted
Para ficar assim:
deb http://us.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
deb http://us.security.ubuntu.com/ubuntu/ precise main restricted universe multiverse
Em seguida, basta executar:
sudo apt-get update