Heroku PostgresApp - Fails to start with edited postgresql.conf
This is probably a kernel resources issue. Check the PostgreSQL log files to see for sure. They should be in the pg_log directory, alongside postgresql.conf.

The PostgreSQL documentation on kernel resources has a section on increasing shared memory limits for Mac OS X.

Note that increasing some settings in postgresql.conf also requires you to increase shared_buffers. This is usually documented in the info about that configuration option on the PostgreSQL site, and you'll be told in the PostgreSQL logs if you need to increase shared_buffers.