Totem

Please read https://help.ubuntu.com/community/RestrictedFormats. Towards the bottom there is a command that needs to be typed into the terminal which can be found by going Applications > Accessories > Terminal.

If this doesn't work what websites can't you open with totem?

I typed both 'sudo /usr/share/doc/libdvdread4/install-css.sh' and 'sudo apt-get install ubuntu-restricted-extras' but it's still not working.

the website is www.lastampa.it any video in the MULTIMEDIA section on the right hand side

when you typed it did you do 'sudo apt-get install ubuntu-restricted-extras' then 'sudo /usr/share/doc/libdvdread4/install-css.sh'?
I assume you are using firebox to browse the Internet if so try typing.
sudo apt-get install mozilla-mplayer
Also when on the page there is a little grey lego brick in the bottom right click that and select the only item on the list. Then change the drop down box to Mplayer plug-in for mozilla and then close and open firefox again. It does take 5 seconds to load the video at least in my case.

For me it is still not working. The Mplayer plugin starts but the video doesn't play and there's a message saying connecting to server media.lastampa.it.

if you wait it should connect mine doesn't take very long but I am on a fast Internet connection a slower connection may take a long time. It has to load up enough of the video so that it isn't all jumpy when you start playing it.

Along with https://help.ubuntu.com/community/RestrictedFormats (thanks Unbuntujenkisn) https://help.ubuntu.com/community/Medibuntu may also be helpful

OK