What is Flex, and what does it have to do with PushButton Engine?
A while ago, Adobe released Flex Builder, which is an Eclipse-based IDE for building Flash applications. It is more programmer oriented than the Flash CS4, and includes a bunch of libraries (in the mx.* namespace) for building applications in Flex Builder. Recently, Adobe released the open source Flex SDK, which includes the Flex libraries as well as an open source ActionScript compiler toolset. PushButton Engine supports Flex Builder (the IDE) as well as the open source Flex SDK. You can use PushButton Engine with the mx.* libraries, or just with normal Flash APIs (ie flash.*). Check out the Getting Started guide for information on how to use them with PushButton Engine.