1) Drag the 'On Start' block onto the space (From basic).

2) Drag 'radio set group' from radio into on start.

3) Drag 'on button A pressed' onto the space (from Input).

4) Drag 'send radio string' into on button a pressed from radio and type in your message.

5) Drag 'on radio string received' onto the space (from radio)

6) Drag 'Show string' from basic into on radio string received.

7) Drag 'Received string' from the block on radio string received 

into the space of show string.