Install MongoDB with Homebrew

$ brew install mongodb
$ mkdir -p /data/db

Set permissions for the data directory

Ensure that user account running mongod has correct permissions for the directory:

$ sudo chmod 0755 /data/db
$ sudo chown $USER /data/db

Run MongoDB!

$ mongod

Note: If you get something like this:

exception in initAndListen: 10309 Unable to create/open lock file: /data/db/mongod.lock errno:13 Permission denied Is a mongod instance already running?, terminating

It means that /data/db lacks required permission and ownership.

Run ls -ld /data/db/

Output should look like this (willis7 is directory owner and staff is group to which willis7 belongs):

drwxr-xr-x  7 willis7  staff  238 Aug  5 11:07 /data/db/