Flixel is designed to work with a variety of cheap or free Flash development tools. FlashDevelop and Flex Builder are two of these tools. FlashDevelop only runs on Windows. If you use FlashDevelop, you're going to need to download Adobe's free Flash tool Flex 3 SDK v3.4. If this sounds crazy or intimidating, try this handy tutorial.
Step 2: Download flixel
All you need to do now is actually download flixel. Note for pros: the flixel source is hosted on github! Download flixel by clicking here.
Step 3: Make a Game!
Hello World is always a good place to start. Here are versions for Flex Builder, FlashDevelop, and MXMLC. You can also check out the official flixel wiki or join the flixel forums to both give and receive help and advice. Finally, these sample projects all have source code available:

Step 4: Share it
The best thing about maing a Flash game is that almost everyone with a computer can play it. Upload your finished game to SWFCabin so you can share it with the rest of the world!
Display Hundreds of Objects at Once
Create Tilemaps from Text Files or Images
Generate and Emit Particles for Hot Effects
Play Positional, Streaming, Looping Sounds
Scroll Objects or Groups in Parallax



