See http://wiki.scummvm.org/index.php/Datafiles for files needed fro specific games.
Copy files to mmc card somewhere (like scummvm/game/). In ScummVM then click "Add game" button and navigate to /media/mmcX/scummvm/ , select game folder and click "Choose". In 'mmcX' the x is 1 or 2, depends on which card (external/internal) you use.

For tablet specific issues check also http://maemo.org/community/wiki/scummvm/, for general issues (like how to copy and even compress games, MI3 is large and compression can cut disk requirements to ~500MB) check main scummvm site http://www.scummvm.org/, they also have forums there.