The sec plus license can be purchased from any authorized Cisco reseller, you can google it as asa5510 sec plus license.

Once purchased they will ship you product authorization key known as PAK , once you get that package with this information then you need to go to Cisco license registration page to enter the PAK key and ASA5510 serial number, you will then get the license key by email immediately for you to enter in the ASA5510 and upgrade it to sec plus . When you enter the key in ASA you will need to reboot. Also always save the output of "show version" to keep it in your records prior to entering new key upgrade.

You will need part number ASA5510-SEC-PL

http://www.cisco.com/en/US/prod/collateral/vpndevc/ps6032/ps6094/ps6120/prod_brochure0900aecd80402e36.html

If you have cisco CCO access this is the link to enter PAK and ASA5510 serial number and other necessary information etc..

Try accessing this link , if you get CCO login place your credentials .

http://www.cisco.com/go/license

If no luck with CCO access to above link please call TAC services and ask them how can you register the license after when you purchase the PAK.

http://www.cisco.com/techsupport/contacts

Rgds

Jorge