The "AD Role Holders" report has some incorrect description information on how to run the report. If you follow the instructions... adding the "Active Directory Domain Controller Computer Role" you will get no data returned. This will be fixed in an upcoming ADMP update... but in the meantime:
Instead - you need to add the "Forest" object as a group:
Open the report.
Select "Add Group"
Search for a domain name of the forest root.
Sort by "Type" and add the object for "Active Directory Forest"
Run the report.
This will give you the RID Master, Schema Master, and PDC Emulator role information for the forest.