First, you can enable DCV in OpenStudio:

1. On the AirLoop, click the "Control" button at the top of the screen.
2. Click the Demand Control Ventilation switch to "On." (In E+, this corresponds to setting the Controller:MechanicalVentilation field DemandControlVentilation to Yes)
3. On the AirLoop, click the "Layout" button at the top of the screen.
4. Click on the OutdoorAir System
5. Set the 'Minimum Outdoor Air Flow Rate' in the Controller:OutdoorAir to 0. Otherwise the DCV won't be able to drop the flow rate.

For debugging, I suggest requesting detailed timeseries outputs and graphing them in ResultsViewer.