The addon should be registered in the same way as the software - using the license key. See Store Manager Support regarding registration and possible problems. You can reset addon's license in the same way as the main software license.
ICEcat Integration registration process includes the following steps:
After purchase, all customers receive an email notification with registration information. You may find License Number (Regcode) there.
Open "Preferences" and activate the "Addons" tab to see the list of available addons. As you can see in the illustration below, the addon is unregistered now.
3. Click the registration status, the "Plugin "ICEcat Integration" registration" window will appear.
If the license key for registration is correct, the "ICEcat Integration" addon will be successfully registered.