I recently purchased GPO and found myself unable to install it. It required me to register (so that it would remain working after 30 days) and both the manual and the .html documents told me to access the Start Menu and click on the Kontakt Player Folder, which did NOT appear in the Start Menu. They said a link to the registration site was in the actual DVD-ROM, but it wasn't. The install setup already asked for the ID code I do not have.
I don't know if it's the same on the Windows platform, but on a Mac, you will find a little ap called Registration Tool in the Garritan Personal Orchestra folder. This is the tool you use to start the registration process. You will need the serial number of your version for the Kontakt Player. If I remember, it's been a couple of years now, it is either on the inside of the package or on the manual. You type this number when asked. You will then be taken to the Native Instruments site where you will give them your information, name, email, etc. They will then email you an activation code, which you will use when you first run GPO.
It took me longer to type this than it will for you to register.