http://gronmayer.com/it/dl.php?id=141

Open that link on your N800.

It's an Install file that adds the maemo extras-devel repository (development, where all the alpha and beta stuff goes) to your Application Manager. Normally this would be the end of your troubles and you'd just have to refresh the repositories and install mplayer, but as there are still some outstanding issues with the maemo repositories (******ed ones, mostly ), you'll have to follow these extras steps:

1. After using the Install file above to add extras-devel, go to Application Managers application menu.
2. Then tap on Tools ->
3. tap on Application catalog...
4. Scroll down until you find the "Maemo Testing Extras" repository in the list.
5. Double-tap it, or highlight it and tap Edit.
6. Change the Web address filed from repository.maemo.org/extras-devel to repostory.maemo.org/extras-devel
7. Tap OK
8. Tap Close
9. Tap OK (to refresh the repositories)
10. Install mplayer

Edit: Repositories have been fixed. Use the normal steps.