The registrar-web can be accessed via https://secure.registry.sx/registrar/. Use your login credentials that you used when you applied to become an accredited .SX registrar (login starts with "u").
In case you would have forgotten your password, you can use the Forgot your password?" link to reset your password.
Filling out your login ("u" + digits) and a captcha will let the server send an e-mail with recovery link to the email address originally entered when creating your account.
That link will bring you to a page where you can enter a new password.
Once logged-in you see a menu on the left hand side, from where you can access everything you need to manage your account, contacts and domain names.