Once you bought a plugin in our shop, you get automatically a license key for them. You can see them by clicking on "My account" in the upper right corner, then you need to select "My instant downloads" on the left menu. Now you can see all of your bought plugins and, of course, your license key.
There are two possibilities to enter your license key:
1a) There is a "Enter the license key" button in the lower right corner of the notification dialog, which will open up each 100 IDE starts, or project switches.
1b) You can go into the plugin settings page. There you can enter your license key as well.
2) After you opened the dialog, you can just paste your license code in there.