The majority of people will be able to install Creately Desktop simply by visiting: http://creately.com/diagram-products#desktop and clicking on the Install Now button.
Adobe Air Required
Creately Desktop requires Adobe Air to run. If this is already installed on your system then Creately Desktop will just start downloading and installing. However occasionally it doesn't install automatically. Is this case you should visit: http://get.adobe.com/air/ and download Adobe Air for your operating system and install it before trying to install Creately Desktop again using the Install Now button.
Minimum Version of Adobe Air
Creately Desktop requires Adobe Air 2.5 or above to work. You can check which version you have installed by using the instructions here: http://helpx.adobe.com/air/kb/determine-version-air-runtime.html Normally Adobe Air will update itself when it is run. If you find you don't have the latest version of Adobe Air installed you should install it by downloading it from here: http://get.adobe.com/air/
If you still have issues installing and you have Adobe Air installed you can try downloading Creately Desktop install package manually and the install it from your local desktop. Visit: http://creately.com/desktop/releases to download the latest version or a historical version if required. Once downloaded you should be able to double click / run the downloaded file to start the installation.