A Common Wire, or C-Wire, carries 24v from the heating system (low voltage) to the TSTAT.
If you have one, you should have it running with the rest of the wires for your thermostat.
You can refer to the installation guide below to see how to connect the C-Wire to the TSTAT.
If you have a C-wire, but its not connected to the TSTAT, you will need to remove the TSTAT from the Z-Wave network and wire up the TSTAT. Then reconnect to the Z-Wave network. The TSTAT must be connected to C-Wire first before it is connected to the panel in order to incorporate the C-Wire.
Be sure to turn off all heating/cooling power (usually at the circuit breaker) before wiring the TSTAT.