You can follow this sample config:
http://www.cisco.com/warp/public/110/cvpn3k_pix_ias.html
Note the commands:
aaa-server partnerauth protocol radius
aaa-server partnerauth (inside) host 172.18.124.196 cisco123 timeout 5
crypto map mymap client authentication partnerauth
The "partnerauth" keyword maps the VPN client crypto map to the AAA radius process.