For full details, see the official announcement from the FDT team:
I just gave it a try with the free version of FDT, and it was super easy to get started. I simply added Apache FlexJS 0.5 as an SDK, and then I created a new project using the "Web-JSC" template. Since it included some sample "Hello World" code, it was super easy to launch it in the browser right away! I made a few tweaks to the code and FDT showed all of the appropriate code hints. Great stuff!