if you're looking for a package use this site:
http://packages.ubuntu.com/
it has mplayer on it as well!
by the way, if you installed ubuntu from the cd and your internet was not active during the install then you'll have to uncomment the mirrors in your sourcelist!
and where it says "universe", you have to add "multiverse" (explanation why:
http://www.ubuntulinux.org/ubuntu/components)
(this might be handy as well:
https://wiki.ubuntu.com/MplayerInstallHowto)
