Switch checkbox default to checked

Hello I have looked through the forum but the only similar post I could find did not receive an answer. I worked with a web designer but now I’m left to fend for myself, and doing as much research as I can. I’m not sure where to find the “useage popover” but I did try to find it for information…

My web designer created several custom contact forms that have checkboxes left clear as the default. I’d like to change the default to having them checked off.
I can find the section with these options but can’t figure out how to change them:
slug=joinlist label=join our email list value = 1 (is dead: no)
slug=nolist label=No value = 0 (is dead: no)
slug=yeslist label=Yes value = 1 (is dead: no)

If any of you brilliant and oh-so-helpful wizards out there can help me figure out how to get those checklists set to YES (checked off) as a default I will be so grateful. I’m not that familiar with code or where to put that, but I’m trying to learn…(or find someone to help me input that if needed)

Thanks in advance for any advice.

(also I know the WordPress version is 3.3.1 but the web designer warned me about upgrading “the site is working just fine, don’t risk having anything not work by upgrading” so I’m afraid to upgrade)…

http://ww.wp.xz.cn/extend/plugins/custom-contact-forms/