1) First you have to get the reCAPTCHA public and private keys :
go to https://www.google.com/recaptcha
and signup with your @gmail google account
---> "Add new site" inserting your domain name ---> create
Now you have a public key and a private key associated to your domain.
Note them down somewhere.
2) On your q2a website Admin panel, select
Admin ---> spam ---> "Use captcha for anonymous posts"
3) click on
Admin ---> spam ---> "Use captcha module" ---> reCAPTCHA ---> options
4) insert into the 2 textfields the public and the private keys you've got at step 1)
---> Save changes
Now a captcha should be enabled for anonymous posts