Since SANDBOX mode is a different system, it must be registered for separately than LIVE mode. SANDBOX may be used to test creating HITs once properly registered (your HITS will not be answered by workers). Your HITs are viewable on the MTurk site at http://requestersandbox.mturk.com where you can register for SANDBOX mode. Sandbox mode sets your account balance to a fixed $10,000 (this is not real money).
For your security, once entered, the Secret Access Key will not be visible to you. You may wish to store it in a safe place.