It is not that easy to connect AppInventor with your smart phone.
I had to hustle with three computer to succeed.
I checked with three Browser:
Opera: sometimes works, sometimes not. I can’t find out why.
Firefox: Does not work at all.
Chrome: Works always (would be a shame if not)
Be patient, it sometimes takes a minute to connect.
Hint: Activate the USB debugging mode in your phone and don’t shake it too much, because the connection will get lost when the sensors try to flip the screen from horizontal to vertical mode.
If you still fail to connect try youADB. This is a Windows tool which allows you to do various actions by just a couple of clicks. It will help you to establish a connection via ADB (=Android Debugging Bridge) which is essential for AppInventor. Go to „Phone|ADB“ and press „Remount“
Still trouble? Than try adding „adb.exe“ to system path.