Using max with Synthesia

Synthesia is an animated piano-roll notation software in the style of guitar hero, but for piano. You can see how it looks like in the video below.
https://www.youtube.com/watch?v=2bRkE9MbCqM

In synthesia the keyboard is plugged to the computer via a MIDI/usb cable.

What I want to do is to use Max to change the pitch of the notes I am playing on the keyboard and then send the information to synthesia. There cannot be any noticeable lag in the process.
I want to do this because I want to read all the notation in the key of C major, but I want to play the notes and hear the notes I am playing in the original key.
As an example:
Let's say the song originally is in D major.
Then I would play the song normally in the key of D major on my keyboard.
I should also hear the notes I am playing without any transposition, that is, in D major.
As I play, the unaltered notes would be sent to Max. In Max the notes should be altered. In this case Max would subtract 2 semitones from every note (because the distance between C and D is 2 semitones).
Then Max would send each altered note to synthesia in real time as I play them.

The part of creating a patch that subtracts the semitones from every received note I can do without a problem. But I don't know how to set synthesia to receive the information from Max. Also how to set the computer so that the information will always pass through Max before going to Synthesia.

Any help will be much appreciated.

Thank you,

Fernando