Firstly you have to check if you have a MIDI input to your PC. They don't tend to come as standard.
If not you need to get a MIDI to PC interface. They can start at £20 for a MIDI to USB piece of wire up to £100's for a specialist PC card. It's all down to latency ( the software delay between hitting a pad and the sound being played by the software )
Having got a MIDI input you then need to make sure your Drum software is being triggered by the external input. Also, but I'm sure it has, a MIDI output from your drummers kit. I don't know about your software specifically, but it will probably have something under a "Settings" or "preferences" menu which will switch this facility on.
If all else fails, Google to the rescue. Almost certainly someone else has had the same issues as you and posted a solution somewhere.