To enter your license key, you need a mySteganos account. If you have not created one yet, please do so here: mySteganos
Once you have your account set up, please log into it in your preferred browser. You will end up with this overview:
Choose the product you want to activate and select "Enter serial". If you already have an expired license for said product, please click on "Your license" and then "Enter serial".
Now just enter your license key and click on redeem.
To use the license, just start your product and enter your login credentials when the login mask appears.
Steps for version 21 or older:
You can activate your serial number in the welcome screen which will open every time you start or quit the trial version.
There is a link on the bottom left called "I have a serial number". After clicking on the link, you can enter and activate your serial number.