If Morphestra doesn't show up in Service Center, you'll need to copy the
Morphestra.xml into your computer to be able to register with NI through
Service Center. Please make sure you activate the library from Kontakt in
stand alone mode NOT from within a host (like Cubase, Logic etc).

First download the Morphestra.xml (Download) and unzip:
http://www.samplelogic.com/Morphestraxml.zip

The "Morphestra.xml" file needs to go into the Service Center folder located
at

On a PC: Program Files/Common Files/Native Instruments/Service Center
On a Mac: Library/Application Support/Native Instruments/Service Center

Then please relaunch Service Center to activate and also make sure you restart
Kontakt to verify that Morphestra is working as well.