Here is my experience with this activation issue.
The reinstall option only did not work for me, as there is no uninstaller for the PMDG Operations Center.
What worked for me was;
1) uninstall PMDG 777, leave the Operation Center as it is.
2) with a RegCleaner, clean all the old FSX entries, including the FlexNet Licensing Service entries! (as PMDG used this with FSX)
3) reboot
4) install/run SimConnect
5) install PMDG777, start P3D and select the PMDG777
6) there should be an Activation Window now..
I hope this also helps for you guys.
Ardjoen