(I don't have CS5.5 myself, or a Mac, but have set it up on a mates machine so I know it can be done, please bare with me!)
1. If Flash is running, close it. Go into your Flash directory (usually Programs > Adobe > Flash) and find Common > Configuration > ActionSctipt3.0
In that folder you should see several folders, including FP9 and FP10 etc.
Create a new folder and call it FP11.
2. Download the globalPlayer.swc from Adobe labs.
http://labs.adobe.com/downloads/flashplayer11.html
Save it in your new folder. (it will have a longer name, so rename it to playerglobal.swc)
3. You need to create a player profile, so you can choose Flash Player 11 from Flash.
Under Common > Configuration > Players you will see a bunch of xml documents. Open the one called FlashPlayer10_1.xml or any Flash 10 player file.
In that you will see a path to a playerglobal.swc
Change the path so it points to the one you just downloaded and change the name of the player, etc.
There's 2 lines I changed in my CS5 version:
<player asversion="3" id="FlashPlayer11" version="13"></player>
and
<playerdefinitionpath as2="$(UserConfig)/Classes/FP10;$(UserConfig)/Classes/FP9;$(UserConfig)/Classes/FP8;$(UserConfig)/Classes/FP7" as3="$(AppConfig)/ActionScript 3.0/FP11/playerglobal.swc"></playerdefinitionpath>
(I don't know why it's version 13, but it is!)
Save the file in the same folder, as FlashPlayer11.xml
Here's a link to my file, if you're stuck, but it may vary slightly depending on where you have flash installed.
http://www.videometry.net/broomstick/FlashPlayer11.xml
Now you should be good to go!
Remember to choose Flash Player 11 in you publishing settings and Hardware Settings: "Level 1 - Direct" to turn on hardware acceleration.
When publishing, you need to publish direct to the browser (F12) as Flash will still have a F10 stand alone player for previewing/debugging.
If you want to run examples that use the EMBED tag you will need to download the Flex SDK.
Any questions, or variations you come across, just make a comment and I'll help if I can.
Good luck!
