Drop Down menu needs to be

I need to make a change to the Super Store Finder Script that we purchased from Codecanyon.com. You can see an example of the script at [login to view URL] In the Administrators section under "Add a Store" you are only given the option to choose one category per store. We need to be able to select multiple categories. Basically we want checkboxes instead of a drop down. I need someone to modify the Javascript, PHP and mysql database in order to make this work. Thanks Aaron