If your domain is managed in my.rackfish.com:
Log in to your account and select "Manage" - "My Services" - "Domains" in the main menu if you do not see the domain in question.
Click on the domain and then select "Authorization/EPP key". Then press the button at the bottom called "Request Authorization / EPP Key" and your code will appear there.
If your domain is not in my.rackfish.com, but is managed by Rackfish - contact support@rackfish.com .