update manager fails to find list of sources

Thank you for taking the time to report this issue and helping to make Ubuntu better. Examining the information you have given us, this does not appear to be a bug report so we are closing it and converting it to a question in the support tracker. We appreciate the difficulties you are facing, but it would make more sense to raise problems you are having in the support tracker at https://answers.launchpad.net/ubuntu if you are uncertain if they are bugs. For help on reporting bugs, see https://help.ubuntu.com/community/ReportingBugs .

Could you please paste the contents of the file /etc/apt/sources.list ?
You probably have a malformed entry on line 46.

# deb cdrom:[Ubuntu-Netbook 10.04 _Lucid Lynx_ - Release i386 (20100429.4)]/ lucid main restricted
deb-src http://archive.ubuntu.com/ubuntu lucid main restricted #Added by software-properties
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.

deb http://archive.ubuntu.com/ubuntu lucid main restricted multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid main universe #Added by software-properties

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu lucid-updates main restricted multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid-updates restricted main universe #Added by software-properties

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://archive.ubuntu.com/ubuntu lucid universe
deb http://archive.ubuntu.com/ubuntu lucid-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb http://us.archive.ubuntu.com/ubuntu/ lucid-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ lucid-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
deb http://archive.canonical.com/ubuntu lucid partner
# deb-src http://archive.canonical.com/ubuntu lucid partner

deb http://archive.ubuntu.com/ubuntu lucid-security main restricted multiverse
deb-src http://archive.ubuntu.com/ubuntu lucid-security restricted main universe #Added by software-properties
deb http://archive.ubuntu.com/ubuntu lucid-security universe
wget http://www.getautomatix.com/apt/key/gpg.asc
gpg --import key.gpg.asc
gpg --export --armor 521A9C7C
sudo apt-key add -
sudo apt-get update
sudo apt-get install automatix2

well, your problem is at the end of the file. Please note that the lines

wget http://www.getautomatix.com/apt/key/gpg.asc
gpg --import key.gpg.asc
gpg --export --armor 521A9C7C
sudo apt-key add -
sudo apt-get update
sudo apt-get install automatix2

do not follow the format expected by apt.

So remove those lines editing the file as super user, for example executing
sudo gedit /etc/apt/sources.list

Then save the file, and execute
sudo aptitude update.

After that you shouldn't get the error again.