Hi Hans-Peter,
The details are in the FAQ here
Cut and paste :)
1. Go to Dashboard > Plugins, then click on “New Plugin”, then click on “Upload Plugin”.
2. Select gwolle-gb-addon-2.1.0.zip to upload (or any newer version you bought).
3. Activate the plugin.
4. Navigate to Dashboard > Guestbook > The Add-On and select the options you want.
Does this help you?