From MEPIS Documentation Wiki
To get a DVD to play correctly you may need to add a few more programs than what comes installed.
To use a graphical method, follow these steps:
- Open Synaptic by clicking Menu --> System --> Package Manager, and supply the root password.
- On the menu bar, click Settings --> Repositories. Scroll down to the bottom of the list, and check http://www.debian-multimedia.org/ to add that repository. Click OK.
- Click the Reload button to refresh the Synaptic sources.
- Using the Search box, find and mark for install libdvdcss2 package.
- Install and exit. Now DVDs should play just fine using KMPlayer or another player.