How to: Modular limbs

[JmOz]

I think I am doing something wrong

Trying to add a modular limb to my sam

Full cyberarm, modular connection at elbow. Added that part right, but can't figure out how to add the lower arm part. Logically figure it would be a mod on the modular connector, not an option. Tried to add the lower arm separately but takes from essence instead of capacity. What am I doing wrong?