flixel is a completely free collection of Actionscript 3 files that helps organize, automate, and optimize Flash games; an object-oriented framework that lets anyone create original and complex games with thousands of objects on screen in just a few hours. You can read more about it here.

flixel is not intended to be used with the Flash IDE, since it is a pure Actionscript library. I recommend using one of the following:

Having trouble with flixel? Check out the flixel wiki, and if you still have questions, want to meet other flixel devs, or have games to share with the community, check out the forums!