Search the repositories for OS 2008 www.gronmayer.com/it/
(there is a search feature build into webpage, make sure OS2008 is selected)

This will advise which additional repository you require and it provides a one click install in order to include repository in your application Manger list of repositories.

Then you just need to update packages in application manager and mplayer should be available for download. (note warnings about the repository you are installing on the device)