I recently upgraded to a new machine and Flash Player 10. Accordingly, I had to install a new debug Flash Player and setup my Flash log again. I figured I'd share the well-known setup, for posterity. As with most things on this blog, these instructions are for meant for those using OS X.
First, grab the Flash Player 10 debugger on the Adobe website and install.
Second, create a file named mm.cfg in this location:
In the file, add the following three lines:
ErrorReportingEnable=1 TraceOutputFileEnable=1 MaxWarnings=0
The first two should be self-explanatory, however the final line just allows more than the default 100 errors and messages to be logged.
Next time Flash fires a trace function, a file named flashlog.txt should be automatically created and begin logging output from Flash. Previous to Flash Player 9, you could specify the location of the log with TraceOutPutFileName. However, now it's created in an unchangeable location:
Of course, if you're working within Flash, you can view these messages in the Output window. However, when previewing Flash in a browser or the standalone Flash Player, the Output window is not available.
tail -f ~/Library/Preferences/Macromedia/Flash\ Player/Logs/flashlog.txt
Note: flashlog.txt is overwritten by each Flash movie, so it's contents will be cleared out each time you load up a SWF.