If the green LED is lit, the DAC20 is seeing the DCC signal correctly; if it is unlit, it is not seeing a DCC signal. That means that either track power is off, or the wiring should be checked.

When a DCC command is sent to the board (i.e. to change a point on the board to closed or thrown) the LED will flash off twice in quick succession. This provides an indication that the board address has been correctly set.