I can't find the "output.log" file when running my app on a Mac. On a built player (not editor).
If you mean the editor log file, it's in:
As a shortcut, you can open it by clicking the "Open Editor Log" button, in the top right corner of Unity's console window.
Also in that folder are "Player.log" and "Webplayer.log".
For windows EXE stand-alone builds, the output log will be in the 'Data' folder that accompanies your build. For Mac stand-alone builds, it seems all builds write to the same location mentioned above ("Player.log" in /Users/YourUserName/Library/Logs/Unity).
This seems to mean that each OSX standalone build that you run will overwrite the previous log, even if it's a build from a different project.
When sandboxed (eg: for the Mac Appstore), then the player.log is in this folder :
Duck is correct that the log can be found at:
You’ll need to remember that the Library folder is not visible by default. You can turn off hidden folders with these steps: