By holding down Alt+Shift while the activator application is active, the Cancel button will say M. ID, click the button and you will get your machine id.
Machine ID
To easily acccess the activator application you can, after having installed the plug-in within Adobe InDesign select: Activate Plug-in (or Deactivate Plug-in) found from the plug-in menu and the activator will be opened if you have administrator rights.

Please note that if the cancel button does not show M. ID you might need to click in a random field and try again.