I've finally made some head way on this issue. To get the RADIUS plug in to show up, I went to Configure > Role Based Services > RBS Configuration and noticed that there were plugins showing up as uninstalled and out of date. After installing and updating those plugins, one of which was RADIUS, I was then able to access the RADIUS plugin. On my first attempt to extend the schema, I was getting a SSL connect error. After searching around, I found this TID (https://support.microfocus.com/kb/doc.php?id=7014640)and it fixed the SSL connect error for the RADIUS plugin.

After that, when I clicked RADIUS > Extend Schema, I got "The schema extensions for RADIUS have not been installed. Would you like to extend schema now?". When I click OK, I get the following message

Schema conflict detected. Conflict details: [ ObjectClass Name(OID): rADIUSProfile(2.16.840.1.113719.1.39.42.2.0.10) Conflicts with Freeradius Objectclass : radiusprofile(1.3.6.1.4.1.3317.4.3.2.1) ]
Would you like to continue extending the rest of the class(es) and Attribute(s) ?
I chose to extend the schema, but iManager didn't seem to think that the schema had been extended as I kept getting the same message when I clicked RADIUS > Extend schema.

The instructions on this page say to modify the class map, but since I am unable to do since nothing happens when I click to add a new map class, I ended up going a different route and I was then able to extend the schema. I went to Schema > Delete Class, selected radiusprofile from the list and then clicked ok. After the removal, I then went back to RADIUS > Extend schema and it successfully extended the schema. Now when I click on RADIUS > Extend schema, it tells me
The schema extensions for RADIUS have previously been installed and are up to date.
Now I will proceed with my FreeRadius configuration and hopefully I can continue forward on getting RADIUS up and running in my environment.