Remove the C/O cap and install a male adaptor and bush it down to 1.5". Install a short peice of pipe and a 1.5" san. T. You will come off this T for your trap and stand pipe. On the top of the T add a peice of pipe that will go 4" min., higher, than where the stand pipe stops and install a 1.5" female adaptor. Purchase an AAV (air admitance valve), teflon tape the threads and install it.
You should have a 2" air gap between where the stand pipe stops and where the softner line discharges.
If you needed to this whole set up can be unthreaded to snake the line.