Looking at the data sheet on the input modules, looks like input voltage goes to pins 1 & 2 like the coil if you will, pin 3 gets +5vdc, pin 5 gets common or ground and it should sink pin 4 to ground when the input goes high.
The outputs work more like a relay, common goes to pin 3, when +5vdc goes to pin 4, a triac fires on the AC modules and a transistor on the DC connecting pin 1 to pin 2. If that makes sense. I've never had to think about how they work before.