First, check the target
echo $MIX_TARGET
If it's not the expected value, or it's blank, set the target
export MIX_TARGET=rpi0
Then run mix firmware from the root directory of the circuit to build the firmware.
Finally, setup the hardware, plug in the device and after it has booted (~30 seconds depending on the model), run mix upload to load the firmware onto the device.
The device will reboot and the code from the firmware will be executed.