Use gksudo gedit /etc/apt/sources.list.
Find these lines in that file.
#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
#deb http://archive.canonical.com/ubuntu lucid partner
#deb-src http://archive.canonical.com/ubuntu lucid partner
Now remove the # to include these repositories and save it. The lines should now look like this:
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
deb http://archive.canonical.com/ubuntu lucid partner
deb-src http://archive.canonical.com/ubuntu lucid partner
Run sudo apt-get update and then sudo apt-get install vlc vlc-plugin-* mozilla-plugin-vlc.