SEP and SNAC are licensed separately, and you need a license for SNAC in order to enable that functionality on a SEPM.
What your upgrade notification refers to, is the Symantec Protection Suite, which include licenses for SEP, SNAC (self-enforcement), Web Gateway and a whole load of other products as you can see (everything in that list in fact).
Either via https://licensing.symantec.com or in the license pdf Symantec provide, you should be able to find a separate serial number and slf file for SNAC. This needs to be applied to your SEPM to enable the SNAC functionality.
I often see companies purchase the Suite as it's a better deal, but then only go on to implement SEP and forget about the rest of the products. I'd highly recommend reading up on all of the products you are seemingly licensed for, to see if these can contribute to the security of your environment.
As you've got the licenses, why not use them?