Now i try to launch the s2a_fm soft in the console but it answers me "COuld not instantiate Pymata. Is your Arduino plugged in ?" which it is obviously.
I actually imagine i could try to get the information into snap! then transmit them with OSC to a max patch...
When you invoke s2a_fm, the default serial port is /dev/ttyACM0. If that is not the port that you are using, then you would need to specify it. So, for example, if you are using Windows, and your com port is 4, you would start s2a_fm with:
python s2a_fm.py com4
I am not familiar with max/msp, but if it works with Firmata, then all you need do is load the StandardFirmataYun sketch into the Arduino and you don't need PyMata.
Hope that helps.