Can't figure out "Ability Level Bonus"

I have no idea how the Ability Level Bonus upgrade works, I've tried using it on an ability, but the ability never levels up.
Ability: Slow Aura. 12 Levels.
Upgrade: Upgrade Slow Aura. Ability Level Bonus: Slow Aura, Base 1, Increment 1 or 2 (tried both)

Am I doing something wrong? Can I not use it on an ability a building is using? Please help me.

you have to make whatever units that are using it, use the upgrade. the ability level bonus doesnt upgrade the ability itself, it just adds to the levels of units with that ability that can recieve the upgrade.