How to: Modular limbs
 

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?

There's no rule that says it takes from capacity. A lower arm takes Essence. It doesn't necessarily make sense, but that's how the rules are written.

If you already have a full limb replacement, however, you've already paid the essence for the lower arm. Buying a modular elbow connector should just allow you to swap in/out your lower arm. That's why the Elbow connector costs a bunch of capacity, because it's removing the lower arm portion.

It looks like HeroLab is not currently supporting Modular limbs fully (or at least not correctly). JmOz, I suggest you submit a bug report to make sure it is on the list of things to fix.

The rationale I have is below, but the tl;dr answer is:
Use the Augmentation Essence Cost Permanent Adjustment under the Personal Tab.

Yeah it should already be paid for by the capacity, but does it say that anywhere? Like how a modular shoulder or hip is only 0.2 ESS, but the full limb for it is an entire point, even though you got your arm cut off. I mean, it makes perfect sense, but the weird confusing screed that is the modular limb description doesn't mention it. It seems to be RAW that stuff costs in ESS what it costs unless there's a specific Capacity listed for the augment. And there isn't, so it seems like not a bug because in the absence of a specific rule, the Essence cost stacks, otherwise, you're doing a GM ruling/houserule. It's completely illogical, but it's written that way. Even still, if you plugged a betaware arm in it, you'd lose less Essence even though your arm was still cut off!

Except the essence for the lower arm has already been paid for. When you buy a full arm, it includes the lower arm. The modular connector is only supposed to allow you to swap out that part. It is still a part of your full cyberarm that's already been replaced.

Yes, a modular connector attached to flesh costs essence, and you don't get the rest of the arm for free (essence-wise). Any cyberlimbs that you are using you should be paying for with essence. But when you are just replacing a piece of a cyberlimb you already have, you don't pay the essence cost again.

Essence Cost Modular Limbs
 

Sorry if this is a necro, but the question is floating around on FB atm. Most of the difficulty people seem to be having is not seeing the same kind of active/inactive button that exists for weapons etc. and, like me, not realising you have to do that in the In-Play tab. It's not consistent with how many of the other features work.
As the essence cost for cyber-bits are set, imo any main or off hand limbs/bits after the first should have no essence cost, but managing that in the code would probably be a pain.
There also seem to be people not understanding that the Modular option is down in the modifications for the relevant bit. Perhaps items like Modular Arm in the augment list with the cost of the modularity already applied?