It is possible that other fields need to be filled out as well. The captcha image protects the page against spam robots. You have to write the letters you can see on the image into the related text field. You can recreate and enlarge the image if you have difficulties reading it. If you do not manage to complete that step you can contact an administrator.
With an existing user account you can login to the page. Click on the menu item Login and enter your username and password. If you have forgotten your password you can use the Forgot password function. You will have to enter either your e-mail or your username. If your account has not been activated yet you will have to go to the login page.