There are a couple of things I can think of:
1. You might not have chosen the correct race to gain that ability. Check in the unit details screen or the unit design screen to see if your units have Amarian Blood (which grants the lightning attack) or if they have something else.
2. It might possibly have been removed from the action bar. In that case, you should be able to find it by opening the 'Spells' or perhaps the 'Skills' list (click the button labelled 'Spells' or 'Skills' under the action bar in a tactical battle). If it's there, you should be able to bring it back into the action bar by clicking on the circle with a star in the middle, and then clicking a spot on the action bar, or you can use it from the pop-up menu from the button.
3. If you created a custom faction of a non-Amarian race, played for a while and created a few custom units for it, and then created a new custom faction with the Amarian race but used the same name, there at least used to be a bug where your custom units from the previous version of the faction and would use the old version's race instead of the new one.
And is it something I can add through troop creation, given enough crystal?
Not without modding. This is a bonus for Amarian blood, and should show up at no cost on any units with that blood trait.