Can't set a checkbox with watir-webdriver

I'm testing my app with tumblr and i have to log in and out as I go through procedures. While doing so, I'm having trouble clicking a checkbox that keeps popping up. How can I use watir-webriver to click it.

I've used the

browser.checkbox(:value => "recaptcha-anchor").set

and

browser.checkbox(:id => "recaptcha-anchor").set 

and

browser.span(:id => "recaptcha-anchor").click