i think you shoud frist register in the site
http://feedburner.google.com

this is a way of it but in arabic too
http://www.traidnt.net/vb/traidnt1415596/
you have your code when you finish

then you can open
footer.tpl in your site files
and go down of it
and put the code you have from feedburner down of it


.. before you change enything take a backup from it

the file i say in here
look\styles\forum\main\templates\footer.tpl

open it
and go down and put it there

after that