The Tablet Converter was written for Nokia's internet tablets, and isn't officially intended for Symbian devices at all, but in the real world it seems to work fine with S60 and is much easier to use than the official application.
You only need to do two things really:
1. Drag the video file into the app window, it will start converting automatically
2. When the video has finished converting, attach your phone to your PC with a USB cable in data transfer mode, and click on the video's "copy" icon (the arrow pointing into a box). Your phone's memory card or built-in mass storage should show up as "Removable Drive" possibly with a letter after it.
...and that's it, no need to know about codecs or bit rates or anything like that. It's not the most fully-featured or flexible video converter ever made, but it's possibly the easiest to use.
If you're looking for a free, legal, full-length film which features explosions and stuff to watch on your phone, try downloading the Finnish cult classic Star Wreck: In The Pirkinning, which has possibly the best special effects of any zero-budget film. (Bear in mind that if you download the Xvid version you may need to install the Xvid codec for your PC before converting.)