Launch the main program (sometimes referred to as the layout desiger) and select from the menu:
"Help",   then   "Enter License Key"

You will see a pop up window that looks something like that:





There are two parts to a license key: a name and the key itself. Enter provided information as specified in your e-mail and press OK.

I would like to take a moment to thank you for purchasing this software. Your support is greatly appreciated and will surely lead to further development of great software titles in the future.