You would need to purchase the license from Cisco Partner first. The license is based on the serial# of the FWSM. If you are running FWSM in failover mode, you would need to purchase the same license on both FWSM.
Once license has been purchased, you would need to send email to licensing@cisco.com with a copy of "show version" and the license purchase proof, and the activation key that you can install to your FWSM will be sent to you.
And here is how you enter the activation key:
http://www.cisco.com/en/US/docs/security/fwsm/fwsm32/command/reference/a2.html#wp1474929
Hope that helps.
