SimpleDiagrams comes in two basic forms: a "native" version for Windows or OSX and an Adobe AIR version.
Most people want to install SimpleDiagrams like any other application. If that's you, simply click the download button at the top of the page. The download button should recognize whether you are using Windows or OSX and start a download of the correct version for your operating system.
However, some people like the Adobe AIR framework because it lets them easily install and manage programs on their desktop. If you want to install the AIR version of SimpleDiagrams, select the .air installer link above.
SimpleDiagrams uses Adobe AIR. Please review Adobe AIR's system requirements.
Read about bug fixes and new features here: Product History