Conflicting instructions on external compass installation

I am trying to install an external triple axis magnetometer on my apm2.5 but the instructions say the i2c port uses 3.3 volts and to change the external mag board to 3.3v from 5 volts. Then under the graphic is says change from 3.3 to 5v before attaching it. So which is it supposed to be, the mag board, 3.3 or 5?
Thanks in advance.

The I2C port is a 3.3v port and the external magnetometer should be setup for 3.3v operation.