There should be an option, after scanning to Openbrowser. Use that, then select Install possibly.
or if you scanned the apk qr code, you should see
Http://appinv.us/companion click on that? or load the url into your browser on your Android.
Do you want to use the MIT App Inventor Companion with Gameblox or with App Inventor Sasha? The Companion is for use only with App Inventor.