To be able to run your app in the AppBuilder companion app, do the following:
1) Right-click your project in Visual Studio and select Build in Cloud
2) Select the platform that you want to build for.
3) For the given platform, select the AppBuilder companion app
4) Click Next
When the build completes, you will see the QR code that you should scan to load your app. Note that it will no longer be active once you trigger another build (no matter what OS you choose).