Hey Paul, thanks for your time!
I never even thought about looking into specs of the original Fio you linked as I immediately looked at the docs of the Fio v3 by Sparkfun. As you can see, the Fio's Schematic and Pin Assignments http://arduino.cc/en/uploads/Main/Arduino-Fio-schematic.pdf are different to the ones of the Fio v3 http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Dev/Arduino/Boards/Fio-v30.pdf.
That's how I came to the conclusion where to put the wires and what integer to set on attachInterrupt(). In the beginning I tried to do it like it's described for the Fio - A4 (SDA), A5 (SCL) and D2 (int1) for IRQ - it didn't work. Then I did the research, saw that pins are different but that also didn't help. That's why I'm here now 