Hey there,
I am really sorry for the delay of this reply as it is a weekend for us here.
Based on what i checked on your site, it seems that option did shown up on the comment form.
You can check my screenshot here :
http://take.ms/H7ndD
And this option only shown up if that is guest user.
Once user enter their username, password, email and tick that register option before submit new comment, this user will receive an email for the random password, so he can able to use this random password to login on the site.