edita il sources.list e controlla che siano attive le repositry universe multivese (con feisty e gutsy lo sono per defaults con le precedenti sono commentate).
Se sei su feisty puoi tranquillamente usare il sorcio che stò usando io.

## UBUNTU REPOSITORIES
## -------------------
##
## Feisty (Ubuntu 7.04)
deb http://archive.ubuntu.com/ubuntu/ feisty main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ feisty main restricted universe multiverse
##
## Proposed Updates
deb http://archive.ubuntu.com/ubuntu/ feisty-proposed main restricted universe multiverse
##
## Bug Fixes
deb http://archive.ubuntu.com/ubuntu/ feisty-updates main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ feisty-updates main restricted universe multiverse
##
## Security Updates
deb http://security.ubuntu.com/ubuntu/ feisty-security main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu/ feisty-security main restricted universe multiverse
##
## Backports
deb http://archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ feisty-backports main restricted universe multiverse


## ADDITIONAL LINUX MINT REPOSITORIES
## -----------------------
##
## Cassandra (Linux Mint 3.0)
deb http://www.linuxmint.com/repository/ cassandra/
##
## Romeo (Linux Mint Unstable)
deb http://www.linuxmint.com/repository/ romeo/


## OTHER REPOSITORIES
## ------------------
##
## Canonical (RealPlayer10, Opera, DesktopSecure etc...)
deb http://archive.canonical.com/ubuntu/ feisty-commercial main
##
## Medibuntu (Codecs and extra applications)
deb http://medibuntu.sos-sts.com/repo/ feisty free non-free

deb http://kubuntu.org/packages/kde-357/ feisty main
deb ftp://bolugftp.uni-bonn.de/pub/kde/s...3.5.7/kubuntu/ feisty main
deb http://www.mirrorservice.org/sites/f...3.5.7/kubuntu/ feisty main
deb http://mirror.cc.columbia.edu/pub/so...3.5.7/kubuntu/ feisty main

# Many “random” software: aMule, amsn, mplayer, moto4lin, flashplugin & flashplayer (9.x)…
# Further informations: http://3v1n0.tuxfamily.org
deb http://download.tuxfamily.org/3v1deb/ feisty 3v1n0
# deb-src http://download.tuxfamily.org/3v1deb/ feisty 3v1n0
deb http://archive.technalign.com/ explorer main restricted universe multiverse
## experimental complementary ubuntu-feisty - ubuntu-gutsy
## deb http://archive.ubuntu.com/ubuntu/ gutsy main restricted universe multiverse
deb http://archive.technalign.com/ explorer-commercial main restricted universe multiverse
### Additional FreeSpire repositry
deb http://apt2.freespire.org/CNRUbuntu/ skipjack-feisty main restricted
# deb-src http://apt2.freespire.org/CNRUbuntu/ skipjack-feisty main restricted
deb http://apt2.freespire.org/ubuntu/ feisty-updates main restricted universe multiverse
deb http://apt2.freespire.org/ubuntu/ feisty main restricted universe multiverse
deb http://apt2.freespire.org/ubuntu/ feisty-backports main restricted universe multiverse
# deb-src http://apt2.freespire.org/ubuntu/ feisty-backports main restricted universe multiverse
# deb http://security.ubuntu.com/ubuntu/ feisty-security main restricted universe multiverse
# deb-src http://security.ubuntu.com/ubuntu/ feisty-security main restricted universe multiverse