To enter your License Code, simply double-click on Cythera to launch it, and click on the same Register button you used to purchase the License Code.
Instead of selecting Register, click on the second option - Enter License Code.
You can also access the Register application from within the Cythera folder. Open the Cythera folder, double click on the Register Cythera application, click the right arrow, and then select the second option - Enter License Code.
Enter the License information exactly as it was sent to you, and you are all set!