You should install mplayer from here..

http://maemo.org/downloads/product/OS2008/mplayer/

After that, you can follow the instructions on the page with the link to the program. The conversion will take some time and when the file is converted it will be in the destination directory. Then transfer the encoded video over to your sd card. When it's on the tablet you should be able to start up mplayer, which should find the video file and it should play. This program doesn't go onto your sd card for any reason as it's for pc only. If you are truly stumped about this, then maybe we can exchange instant messenger contacts and I can try to walk you through it. Good luck!