Run the following command first to start the mongo server

mongod run --config /usr/local/etc/mongod.conf

Assuming you installed mongo using Brew.

More information about the mongod process here

EDIT

Try to repair mongo using :

mongod --repair

Seems to have issues with permissions also, try using sudo to start mongod

http://stackoverflow.com/questions/17829158/error-connecting-to-local-installation-of-mongodb-thru-terminal

[mongodb]child process failed, exited with error number 100