From http://stackoverflow.com/questions/4881208/how-to-put-username-password-in-mongodb

How to setup auth in MongoDB

1) In mongo command line: (let say, set administrator)
  > use admin;
  > db.addUser('admin','123456');
2) Shutdown Server and exit
  > db.shutdownServer();
  > exit
3) Restart  Mongod with --auth
  $ sudo ./mongodb/bin/mongod --auth --dbpath /mnt/db/
4) Run mongo again in 2 ways:
  i)  run mongo first then login.
   $ ./mongodb/bin/mongo localhost:27107
   > use admin
   > db.auth('admin','123456');
  ii) run & login to mongo in command line.
   $ ./mongodb/bin/mongo localhost:27107/admin -u admin-p 123456

* The username & password will work the same for mongodump and mongoexport.