Access codes are provided by schools and educational consultants to families to invite you to "closed" tests that are not open to the public. This ensures only those invited can register. For "closed" administrations, the test administrator will provide an access code when inviting you. You simply enter the access code during registration when prompted and the test date and time will automatically be set.
If you have not been invited to a "closed" administration, you do not need to worry about an access code. You'll simply select an available test administration during the registration process.