Normally you don't worry about the registration as the installer manages this for you. The problem comes when you have to reinstall notes and forget that you had Google Desktop installed. Not wanting to reindex all your mail again, how do you get that plugin back into Notes?
First locate your notes folder. For me its C:\Notes but it could also be C:\program files\Lotus Notes\. Now locate the notesgdsplugin.dll. You might have to snag this from a coworker if you can't find it anywhere on your computer. Copy this file into the root of your notes directory.
Now locate your notes.ini file and add the following under the [Notes] section. Change path_to_notes to match your installation directory. Restart Lotus Notes and Google Desktop should alert you that its now installed.