Since the Pod HD Pro has no cd/mp3 input, and the iPad has no usb port, I think you will have to connect the iPad stereo output into the LINE L and LINE R pair of inputs on the HD Pro. You will need an adapter to do this.

Then, in the HD Pro, set up a dual path preset. For Input 1, select your Guitar. For Input 2 select LINE L + LINE R (I presume that's an option - not 100% sure. You may need to experiment with your options here). In the signal chain, do not place any amp or FX before the path split so that you keep the incoming signals completely separate. Use Path A for your guitar processing. Leave Path B empty to allow your iPad signal to pass through unprocessed.

I think that should do it.