From the Knowledgebase

In the Premium version there is nowhere to enter the license key

In you admin area, on the Settings > PDF Embedder page should be a "License" tab, and there you can enter the license key you received in your purchase email.

If there is no "License" tab, then you are almost certainly have only the free version activated. On the Plugins page, please activate the Premium version if it's present, or follow the instructions to install the Premium plugin you have purchased.