How do you use the trackpad to draw Chinese characters?

Look at the Help for the Pinyin Chinese IM (toward bottom of flag menu when you have pinyin active)

Your machine must have Multi-Touch for this to work. It was only introduced in 2008.

In system preferences goto language and text, then input sources.
Ensure you have Chinese enabled in the list, also select show input menu in menu bar.

Once you have done this a little flag will be displayed on the menu bar by the clock.
Click on this and it will give you the option to display the trackpad handwriting box.
It will take a little getting used to but works well.

Which program should I be in to get it to work

If you are not familiar with normal Chinese input, here's a whole site devoted to it:

http://www.yale.edu/chinesemac