cannot play dvd's on my laptop

If Vojtěch Trefný answer desn't solve please also try....

Play mp3 and dvd under Ubuntu install skype googleearth acroread and other stuffs using Medibuntu

First please install https://help.ubuntu.com/community/RestrictedFormats

You need to have extra repositories enabled..

Please first enable the universe and multiverse repository:

Open System → Administration → Software sources → [ Tab Ubuntu software ]

enable "Community-maintained Open Source software ( universe )"
enable "Proprietary drivers for devices ( restricted )"
enable "Software restrictecd by copyright or legal issue ( multiverse )"

Close and confirm the repository reload.

Type the following command in a terminal (applications → accessories → terminal)

sudo aptitude install vlc smplayer mplayer

To get better dvd playback and optional packages here the medibuntu available software list http://packages.medibuntu.org/
you need to add the medibuntu http://www.medibuntu.org/ repository to your /etc/apt/sources.list file:
( here the medibuntu howto https://help.ubuntu.com/community/Medibuntu but see below )

In short type or better copy and paste the rows, below one row a time then press enter:

ubu_codename=$(sudo lsb_release -c | cut -d: -f 2 | sed 's/^[ \t]*//;s/[ \t]*$//')
sudo wget http://www.medibuntu.org/sources.list.d/$ubu_codename.list -O /etc/apt/sources.list.d/medibuntu.list
wget -q http://packages.medibuntu.org/medibuntu-key.gpg -O- | sudo apt-key add - && sudo apt-get update

Now to install, type:

sudo apt-get update
sudo apt-get install ubuntu-restricted-extras libxine1-ffmpeg
sudo aptitude install libdvdread3 libdvdnav4 libdvdcss2 regionset gnome-mplayer
sudo aptitude install non-free-codecs w32codecs
sudo aptitude install gstreamer0.10-pitfdll gstreamer0.10-plugins-good
sudo aptitude install gstreamer0.10-plugins-ugly gstreamer0.10-plugins-ugly-multiverse

give your user password when requested, you don't see nothing when you type it, then press enter.

You can also install other Medibuntu repository provided optional software http://packages.medibuntu.org/ :

sudo apt-get install skype
sudo apt-get install googleearth
sudo apt-get install acroread

Hope this helps