If you want to find the location of your log file in Postgres, you'll need to hop into a Psql session..

psql dbname

Then it's as simple as running..

 show data_directory ;

Which will output the data directory, in my case..

/Users/sean/Library/Application Support/Postgres/var-9.5

To see the file name run..

show log_filename ;

There are other useful commands too. You can read more of them here https://www.postgresql.org/docs/9.3/static/runtime-config-logging.html