I'm building a Mendel using Makerbot Gen3 Electronics. To make things easier, I elected to leave everything on the board stock except for adding the I2C pins. My question is, I added the I2C pins (4pins) which are suppose to be connected to D9 D10 (6 pins) on the Extruder board as in the attached photo. Can anyone tell me which 4 pins on the D9 D10 get connected to the 4 pins I added to the motherboard. I am getting better at this..
and thanks in advance.

It isn't I²C, it is just the STEP and DIR signals to the stepper motor.

SDA to D10
SCL to D9

See the Mendel Wiring Diagram.

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Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

I've added your image to the wiki...

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thanks again rh!!