Creately Desktop runs on Adobe Air which is designed to work on Windows, Mac and Linux. A little while ago, at version 2.6, Adobe stopped releasing new versions of Adobe Air for Linux. This means from the official Adobe Air installation page you won't see an option to download Adobe Air for Linux.
Creately Desktop works with Adobe Air 2.5 or later. You can get the Adobe Air runtime version 2.6 from the Adobe Air archives or download the installer directly from here: http://airdownload.adobe.com/air/lin/download/2.6/AdobeAIRInstaller.bin Once installed you'll be able to install Creately Desktop from here: http://creately.com/diagram-products#desktop by clicking on the green install now button.