Cortana is included with the Windows 8.1 "Cyan" update. Does your phone show that particular update is available for the device?
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
DO NOT INSTALL the Developer Preview on the ATIV SE!!!
With out supporting firmware from Samsung you will have many issues running it. Wait for the official release.
And as for "Cyan" Update that Ann refers to, that is only for Lumia phones. Its Nokia's firmware update.