The following are the steps I followed and its working fine for me,
On the AP
1. Setup -> security -> Authentication Server - Make sure that User Authentication is checked.
2. Setup -> security -> User Information -- Add a user
3. Setup -> security -> User Manager -- Enable user manager
On the ACS
1. Setup a group
2. Look for "Cisco IOS/PIX RADIUS Attributes" which will not show unless you have a AAA client
authenticate using the 'radius (cisco ios/pix)'
3. Check "[009\001] cisco-av-pair"
4. Add "aironet:admin-capability=write+snmp+ident+firmware+admin"
5. Add a user in that group