Go to http://www.adobe.com/support/flashplayer/downloads.html and download debug version of Flash Player (both Plugin and ActiveX version).
Close ALL appications (Flex, browsers, ICQ, etc).
Install both players.
Also you may restart you pc.
That is it.
This may be because you use not a debug version of Flash Player.
You can uninstall all Flash Players (it's better to use uninstall_flash_player.exe tool. it's easy to find it on adobe.com site) And after that install debug version of Flash Player from http://www.adobe.com/support/flashplayer/downloads.html (Adobe Flash Player 10 — Debugger Versions (aka debug players or content debuggers).)
If problem still exist try to read http://forums.adobe.com/message/79236#79236
Sadly that's not it.
I already tried uninstalling/reinstalling the debug versions of both Flash Player 9 and 10.
Did not help.
Do you run FB under Administrator account?
Also you can check missing compiler option -debug=false
One of the first things I tried was to explicitly run FlexBuilder as Admin... to no avail.
I set the compiler option -debug=true, does not help.
I do get the "Debugger" and "Show Redraw Regions" in embedded Flash in any Explorer, but I do not get them in my Air Apps!
>> but I do not get them in my Air Apps
it's ok, you shouldn't see this in Air.
Check file C:\Documents and Settings\<you-profile-name>\mm.cfg and remove line (if any) start with PreloadSWF - it may remains after profiler.
You may ensure that you player is exactly debug. Download porcessexplorer from Microsoft. Launch AIR app, open process details, opena tab modules, find plash player dll, watch at it properties, find proreprty debug=1.
If you player is debug version this looks like some bug or Vista blocks connection to player.