A service contract is needed in order to download software from CCO.  The serial number of the ASA should be included in the service contract, and the service contract should be linked to your CCO username.

http://www.cisco.com/web/Downloads/SDS/unentitled_instructions.html

Also, it is highly recommended to purchase hardware from an authorized reseller; that way, you can be sure you have purchased a legitimate Cisco product.

Hope this helps.