The ADU Manager role gives users access to manage ADUs, as well as all employee data, regardless of ADUs. This is a powerful role and should be assigned to as few users as possible.
To give a user the ADU manager role they must also be an HR Admin, as the HR Admin role will give them access to manage employee data for the ADU they are a member of, or any descendants.
All role management is done from the User Management page (HR Manager > Employee Directory > Users).
You will then need to search for the user you wish to add the ADU manager role to. Click on the user's name and this would re-direct you to the user profile, where you will be able to assign the appropriate role(s):
There is no concept of "assigning an ADU manager to an ADU". If you have the ADU manager role, then you have access to all ADUs. If you have the HR Manager role, then you have access to the ADU you are a member of, and/or any below.