To login to classifieds, you must successfully register a username and password. After registering, you are sent a confirmation email. If you have not responded to this email, you will be unable to login.
If you did not receive the confirmation email, check your spam folder.
If you did receive the email confirmation, and you did click on the link in the email and still cannot login, it's possible that text-wrapping in the email prevented a successful confirmation. In this case, go back to the email and copy and paste the entire link (make sure you get it all if it appears to be word-wrapped) into a web browser address bar.
Finally, your username and password are case-sensitive. If you used capital letters when you registered, you must use them to login. Also, there should be no spaces in your username or password.