How do I get the Central Owners (POWL) to show up to assign Mitigating Controls

I have added users in the Central Owners. If I am correct these owners must be added to the target system in order to be seen to be assigned. But now I have made them owners and gave them roles and I still cannot create Mitigating controls because it does not find the AC owner to assign. Please see print screen. Any idea what I am missing to make this happen?