-% mongo
MongoDB shell version: 3.0.7
connecting to: test
2016-02-13T02:10:30.013+0900 W NETWORK  Failed to connect to, reason: errno:61 Connection refused
2016-02-13T02:10:30.015+0900 E QUERY    Error: couldn't connect to server (, connection attempt failed
    at connect (src/mongo/shell/mongo.js:179:14)
    at (connect):1:6 at src/mongo/shell/mongo.js:179
exception: connect failed


[Cannot connect to mongodb errno:61 Connection refused[StackOverflow]](
この通りおもむろに`brew services start mongodb`を打ってみたら無事起動しました。参考までに。