Please start the Online Shield and then click on the shopping cart in the lower left to open the license management screen. Alternatively, you can click on the menu button in the upper left and select "Manage license..."
Now just enter your serial number into the entry field and click on "Activate". A green confirmation should instantly appear and inform you about the duration of your activated license. You can also always check your current license status by opening the license management screen and checking the section "Your current license status".