Found it. It's under the DIChannel.LogicFamily.
using (NationalInstruments.DAQmx.Task task = new NationalInstruments.DAQmx.Task())
{
var chan = task.DIChannels.CreateChannel("PXI1Slot1/Port3/Line3", "", ChannelLineGrouping.OneChannelForAllLines);
chan.LogicFamily = DILogicFamily.TwoPointFiveVolts;
/* rest of code...*/
}