Right now only LDAP is working - RADIUS has an unresolved bug (as of 6.2.0.2).
Here's the BugID:
https://bst.cloudapps.cisco.com/bugsearch/bug/CSCve60272/?referring_site=bugquickviewredir
Here're some related threads about this issue. You need to first setup under System > Users > External Authentication.
https://supportforums.cisco.com/discussion/13267206/fmc-access-shell-external-authentication-server
https://supportforums.cisco.com/discussion/13118331/firepower-shell-authentication-radius