Do you know that your Confluence installation has a license for the Gliffy Plugin, and yet, you find that you are not able to access the Gliffy Plugin?
Gliffy Plugin access is determined by those users with permission to add attachments to a page. Do you have attachment permissions on the page where you want to add a diagram as noted here? http://support.gliffy.com/entries/61904494-Attachment-Permissions-Needed-in-Confluence
If yes please contact us at Gliffy Support
If not, please request permission from your Confluence Technical Contact.