As long as your service contract is properly associated with your ASA and it has the "IPS" in the covered part number, the entitlement is included.
There's no separate license or activation key to use. Signature updates happen by virtue of the ASA serial number being registered with Cisco.
Note that the End of Sales / End Of Life announcement for all the legacy IPS products was put out in October 2014. No new purchases will be sold after next April. Customers are encouraged to move to the ASA with Firepower services Sourcefire technology).