I have found out that by default tfmini driver does not start for certain configurations.
Go to the Mavlink console in qgc and run tfmini status. If it tells you driver not running . Run the following command:
tfmini start -d /dev/ttyS2
Assuming you have it connected to serial 2 port, the driver should start.
Use the analyze tool in qgc, now you should be able to view data in the distance topic.
Also reverify your wiring is correct rx to tx, tx to rx.