The purchases are linked to the primary Google Account on the device on which they were performed and should be restored automatically on any other device using the same primary Google Account.
The primary Google Account is the first Google Account which was used / created on the device. However, in case there are more than one Google account setup on the device, the Google Play in-app purchases mechanism may get confused.
If you are switching devices and are having trouble restoring your purchase, please make sure that on both of your devices the primary Google account is identical or that there is just a single Google account setup on your new device. Then the add-on / license should be restored when you launch the app the next time. In case they are not, please try purchasing the same in-app product / license which you have already purchased previously. The Google Play dialog should tell you that you already own the product and restore it for you before asking for payment.
To mitigate these problems both apps allow you to register with your e-mail. Then your purchases will be stored securely on our severs in addition to Google servers which allows you to restore them from our server even if the primary Google account on your device does not match. If you did not register your e-mail, yet, you can do so by opening the "My license" or "Add-On-Features" screen and touching the "associated account" info at the bottom of the screen.