1. Download the SDK from http://www.adobe.com/products/flex/flexdownloads/

(click on the "I have read the Adobe Flex 3 SDK License" checkbox, and the download link will appear below)

2. Use stuffit expander / unzip to uncompress the archive.

3. Open the terminal (if not open already), and go to the folder where you uncompressed the archive to.

4. Type the following:

sudo cp -r flex_sdk_3 /Developer/SDKs/

5. Edit your ~/.bash_login to include the following line:

export PATH="/Developer/SDKs/flex_sdk_3/bin:$PATH"/

This will add the binaries to the path. If this file doesn't exist yet, go ahead and create one. Next, create a new terminal window to have the right binaries in your path.

6. Try it out. Create a file named "helloworld.mxml" and paste the following into it.

 
<?xml version="1.0"?>
<!-- mxml\HellowWorld.mxml -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
    <mx:Panel title="My Application"
        paddingTop="10"
        paddingBottom="10"
        paddingLeft="10"
        paddingRight="10"
    >
        <mx:Label text="Hello World!" fontWeight="bold" fontSize="24"/>
    </mx:Panel>
</mx:Application>
 

Make sure there is no whitespace before the XML declaration 7. Try it out. Go back to your terminal and type (from the directory where helloworld.mxml is) type the following:

mxmlc helloworld.mxml

This will compile the file using the Flex 3 SDK. It should create a file in the same directory called "helloworld.swf".

8. Open up helloworld.swf in Firefox. If you have Flash 9 installed, it will show you your compiled Flex application!