xbmc is only available in the universe repository, only since Ubuntu 12.04 (precise). If you have an older release of Ubuntu, you'll need to upgrade or to get xbmc from another place.
If you have Ubuntu 12.04, make sure that you have turned on the universe repository. You can see what repositories you have enabled in the file
/etc/apt/sources.list (and in files in the directory
/etc/apt/sources.list.d). Check that this file contains a line like
deb http://al.archive.ubuntu.com/ubuntu/ precise universe
deb http://zw.archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse
If you only find a line like
deb http://mn.archive.ubuntu.com/ubuntu/ precise main restricted
and no line with
universe at the end of that line, or a separate line with
precise universe, as illustrated above.
If you need to modify
sudo apt-get update afterwards, then try installing again. If the package is still not found, post the complete content of
/etc/apt/sources.list and the output of
sudo apt-get update.