Setting up Flash/AIR to log trace messages to a file

Create the flashlog.txt file
Well, let Flash do it for you.

If you have Flash installed:
Go to wherever you installed Flash (usually C:\Program Files\Adobe\Flash CS4) and open the Players\Debug directory. Run the FlashPlayer.exe file. That’s it.

If you don’t have Flash installed:
Run the flashplayer_10_sa_debug.exe file you downloaded above. Again, that’s it.
This step is required because the Debug Standalone Player is what initially creates the flashlog.txt file where all of your trace() messages will end up. To verify that it’s been created, look for it (in Windows) in C:\Documents and Settings\[username]\Application Data\Macromedia\Flash Player\Logs or the equivalent if you have some different install scheme.

Reference:
http://www.thoughtlabs.com/2008/12/09/setting-up-flash-cs4-to-log-trace-messages-to-a-file/