Hello cookieandkate,
I'd only add a little background for future reference to Helga's good advice! (Who by the way is an absolute angel of a helper!)
You can do this by hooking onto one of Thematic's many hooks. Assuming you're new to this (apologies if you're not) Thematic comes with lots of hooks where you optionally add content / markup etc to. These hooks are like pieces of velcro and if you enter the right code you hook onto these velcro hooks markup and content.
One way you can do this is to add your code to one of the velcro 'hooks' that are located before the end of the body tag. In thematic's case this is called "thematic_after;"
So your code might look like this:
// Action hook for placing code before end body tag
function webmagic_code_beforeendbody() {
echo('INSERT YOUR CODE SNIPPET HERE') ;}
add_action ('thematic_after', 'webmagic_code_beforeendbody');
The "webmagic_code_beforeendbody" with a unique name, perhaps something like 'webmagic' (sub in your own name) to ensure it doesn't conflict with any of wordpress native commands.
There are many other velcro type hooks in the footer, for example. To find out where the hooks are have a look in the Thematics "footer.php" and other files, you'll see them there.
**You'd place the code snippet above in your child theme's functions.php file. Child themes are the way to go as your site can update but retain all your unique customisations.