Step 1: Get yourself a Subversion client. My favorite is TortoiseSVN, although that’s Windows-only. There’s a free version of SmartSVN that’s cross-platform, and I use that on the Mac. Or you can go barebones with the command-line client,
svn, from the Subversion site.
Step 2: Point your client at http://opensource.adobe.com/svn/opensource/flex/sdk and do a checkout. If you use this URL, you’ll actually get three branches:
* trunk – where we’ll develop Flex 4
* branches/3.0.x – where we finished up Flex 3 and will develop Flex 3.0.1
* tags/126.96.36.1997 – the code that built the release we just shipped
If you want only one of these, you can check out just the piece you want.
For more info, see the instructions here on our open-source project site.