Enable backend path hints to find out which file it is. You can do it this way.
http://www.classyllama.com/development/magento-development/enable-templateblock-hints-in-admin-panel
Please note this is basically hacking. A better way to do it would be to follow something like this http://www.marketingadept.com/blog/2014/01/magento-developers-add-a-custom-field-to-the-category-admin-page/
It's up to you, speed or do it the right way. Goodluck