The Tor dialog (above) says it will create a log, but does not. Deleted Tor and reinstalling leads to same error.
Currently there is a bug "6.0a5 is not starting on OS X if put into /Applications" in Tor browser. This bug is fixed and after the next release it should be working again.
If you put the Tor Browser on the Desktop or any other place where you have write permissions it will also work. The ticket also suggests if you run
tor.real from the commandline it works also.