Most users here do not have arduino devices. What others in the forum asked and how some of them solved issues is here:
https://groups.google.com/forum/#!searchin/mitappinventortest/arduino It appears no one with a device or experience is around to test your code. There are plenty of posts around talking about how users connect to the arduino.
You could search in the forum using the Search for topics box to the right of the colorful google at the top of this page using terms like arduino or bluetooth or search for app inventor arduino or bluetooth on the Web and found other links.
I do not know why your app fails. I hope you can figure it out with the suggestions above. Perhaps Taifun's suggestion would work? Otherwise, you are going to have to find a solution by experimenting; trial and error.
Best luck. When you find a solution, please return and explain to us what the issue might have been.
Regards,
Steve