The TestResults.io designer will only work with the TestResults.io engine. The engine is required for the extension so that the the test cases can be executed.
You can check which engine is currently in use in the Options screen:
The TestResults.io Designer extension periodically checks if a new engine to download is available. If that is the case a notification is shown on the top bar of Visual Studio as follows:
Example engine version
In order to trigger the installation of the new engine simply press the Install button present on the notification.
If you wish to manually check if a new version of the engine is available simply select the Scan for new engine version button in the General tab.