Arduino Serial Communication with andriod

I want to have an interface between my Arduino board (uno R3) and my ardroid phone using usb serial communication , I am using App inventor 2
I tried these extension

I tried to do as the page but I couldn’t make it work , so if anyone have example for it , even for the block code or even another method.
I am using arduino Uno R3

serial.PNG

This is an Android problem. Most of the USB serial apps on that platform don't work. Serial communication with Arduino is easy. The only Android apps I've found to be consistently adequate are a couple for Bluetooth terminal.

Your be better of with a Arduino mega ADK which is specifically designed for use with android...
arduino-mega-adk-rev3