Adding Join Now! Button to Checkout Page

Hello There is currently a checkbox “Keep me informed of specials” on my checkout page. I would like to replace this with a button that says the same and is linked to my MailChimp sign up form. I thought I would find this in my blocks/wrappers/mainbox_general.tpl but I don't see much there. I prefer to do it this way rather than buy an add-on. People have to opt-in and I have several lists for them to choose from. Thoughts on how to do this is appreciated.