Just like the good doctor told you back in post #4, you'll also need to program those pesky registers. You might have to stare at them for a while. I did. You can either modify them in your program or edit wiring.c. In either case, just recompile your sketch and whatever you edited in the core will also get recompiled.
You do know, of course, that the I2C hardware is for slave only. If you want your 841 to be an I2C master then it is done with a software I2C library and you can use any two pins.