You can auto-update the signature (from Cisco.com) if your AIP-SSM is running 6.1.x. You need a have a valid Cisco Services for IPS account for your IPS sensor. Older versions only allow auto-updating from a HTTP/FTP server running on your network (on to which someone has uploaded updates after manually downloading them from cisco.com).

Have a look at this:

http://www.ciscosystems.com/en/US/docs/security/ips/6.1/configuration/guide/idm/idm_system_images.html#wp1045915

Regards

Farrukh