If you want to restrict the ability to post ads to registered users only, go to your classifieds admin options page under Registration Settings and check the option

Require user registration?

With this option checked users will have to login first before they can place an ad. The login used will be the normal WordPress login. If not logged in the user will be taken to the login form where there will be a link for them to register in case they don’t have a login user name and password.

If your registration and/or login page do not use the default wordpress login (wp_login.php?action=register, wp-login.php?action=login) you can tell the system the url of your registration page and the url of your login page.

**Important: Users will only be able to register if you are allowing registration from your WordPress general settings