Hello Dan,
You need to add the radius client to the IAS ( Internet Authentication Sevice)
Add a client to your radius – In the IAS MMC, right-click on the “Radius Clients” branch and choose “New Radius Client” Enter the Display anem and IP address of the device, click next. Change the Vendor to “Cisco” and enter your shared secret (keep a note of this for later)
This link might help you:
http://briandesmond.com/blog/how-to-authenticate-against-active-directory-from-cisco-ios/
Regards,
Remember to rate all the helpful posts, that is as important as a thanks
Julio
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC