For mplayer, open your 770 browser and enter this address:
http://mplayer.garage.maemo.org/mplayer.install
It should bring up the download dialog box and you can click "Open" to install right now, or "Save" to save the file for later installation.

The Media Converter application is installed on your PC. It is used to convert various video formats into files that can be transferred to the Nokia for viewing on the tablet device.