To enter your License Code, simply double-click on Barrack 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 Barrack folder. Open the Barrack folder, double click on the Register Barrack 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!