Administrator users are not able to manage sites, they do not even have access to the Sites section in HoriZZon if Administrator or System Administrator is their only role. It may however be necessary to manage sites.
Managing a HoriZZon site requires that a user has the Lead Designer role and is the creator of the site. If a Lead Designer user is not the creator of the site, he or she can manage the site if the user has been invited for the model package or project the site is based on.
If you are an (System) Administrator user and want to be able to manage sites, you can use the following workaround to make it possible.
Now you should be able to see all sites based on these model packages and projects. By clicking the in a site you can manage the site, like changing its scope or deleting it.