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