If you have purchased a license and have registered the license but you are still getting prompted to enter a license key then please perform the following steps:
This error is displayed when you are trying to register a license that is for a different version than the version that is installed. When you purchased XrmToolkit you would have received and email confirmation regarding your purchase stating which version you purchased. Your license key is valid only for major versions of that same version. For example, if you purchased v2 then your license is valid for any version starting with 2.
To check which version is installed please click on the 'XrmToolkit' menu item in the main menu in Visual Studio. Then select the menu item titled 'About XrmToolkit'. This will display the version that is currently installed. In the event that you need to install a different version please download the appropriate version from here.
If this does not resolve your issue please reach out to support here.