Umm, i know this may sound silly - but i just see on a pic of the Seeeduino Mega (dont got mine with me here) that the I2C lines are on a place for themselves - are they still set on Analog 4 and 5 or are they 'on their own' now?
On the Seeeduino Mega the I2C are indeed just on their own connector only, no taking over analog input pins.
SDA signal is digital pin 20 and SCL is digital pin 21. There are also ground and +5vdc positions on the 4 pin connector.
Lefty