Problem: The Register Now button cannot be selected. It is grayed out
After opening the Desktop Administrator and clicking the 'Register Single Use and Extensions' folder, the Register Now button on the right cannot be selected because it is grayed out.
This is caused by the user not having Administrative privileges on the machine. Administrative privileges are required to register ArcGIS Desktop.
Solution or Workaround
Log out of the machine, log back in using a username that has Administrative privileges, then run the Registration wizard.
Administrative privileges are only required to register the software, but not to run it.