It is not currently possible to enable a captcha on the login page, and to be honest that sounds like a bad idea to me. No sites I use require a captcha to login. That would be quite a nuisance and discourage use of your site.
I can see where you might like to have a captcha on the registration page and I have a to do list item to make that possible, but currently it is not possible. What we have now is a setting in site settings where you can require that the email address used for registration must be validated before the user can login. When this is enabled, when a user registers it sends them an email with a link they must click to activate the account. Only then can the user login.
Hope it helps,