Activation is done through the Steam platform client, not the Paradox launcher.
Sign in to your Paradox Store account, and select the 'Games' tab.
The product(s) you purchased should be displayed here.
Click on the 'Manage' button of the product you wish to activate.
In the game details that are displayed, there is a STEAM KEY heading and below it there is an alphanumeric code.
Activate the product on Steam with the given alphanumeric code, by following
these instructions.