For installing the latest signatures, you need a valid license. But the sensor will accept system-updates (major, minor, service-packs) without a valid license. These come with the latest signatures of the date when the update is released. So installing the latest version could be a workaround while you wait for the delivery a the services for IPS license.
In addition to that, you can get a tempory IPS-license on http://www.cisco.com/go/license
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