Tracing on linux is hard. The official docs are wrong and multiple blogs contradict each other.
To trace to a file in linux you’ll need the debug player. Copy flashplayer.xpt and libflashplayer.so to ~/.mozilla/plugins. If you use the standalone player, use the debug one. Place your mm.cfg in your home directory. Your flashlog.txt will spawn in the ~/.macromedia/Flash_Player/Logs directory. There is no need to create this directory, flash handles this for you.
Reference: Chris Hill’s blogpost.