As you have the Ardupilot already, don't bother with the MagPi article - that's all about using a Raspberry Pi as the full flight controller - that is possible as the only part of a quad that needs real-time processing is the PWM signal driving the ESCs, and the Raspberry Pi provides hardware PWM - everything else can happen relatively slowly - say 50Hz and there's no need for accuracy in the rest of the code as the processing compensates for any errors in time. My code can easily achieve 200Hz but I limit it to 50Hz to leave space for new function in the future. It's only using interpreted Python.
Now you have the ArduPilot doing this anyway, what do you want the Raspberry Pi to do? Without knowing that, it's hard to say how to connect the RPi to the ArduPilot.
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