Para conectar-se ao mongo, precisamos primeiro iniciar os serviços 'mongod'. Na saída a seguir, você pode ver:
$ mongod
2017-03-05T00:31:39.055+0530 I CONTROL [initandlisten] MongoDB starting : pid=1481 port=27017 dbpath=/data/db 64-bit host=Prabhu-Nandans-Mac.local
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] db version v3.4.2
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] git version: 3f76e40c105fc223b3e5aac3e20dcd026b83b38b
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] OpenSSL version: OpenSSL 1.0.2k 26 Jan 2017
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] allocator: system
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] modules: none
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] build environment:
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] distarch: x86_64
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] target_arch: x86_64
2017-03-05T00:31:39.056+0530 I CONTROL [initandlisten] options: {}
2017-03-05T00:31:39.056+0530 W - [initandlisten] Detected unclean shutdown - /data/db/mongod.lock is not empty.
2017-03-05T00:31:39.057+0530 I - [initandlisten] Detected data files in /data/db created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
2017-03-05T00:31:39.057+0530 W STORAGE [initandlisten] Recovering data from the last clean checkpoint.
2017-03-05T00:31:39.057+0530 I STORAGE [initandlisten] wiredtiger_open config: create,cache_size=3584M,session_max=20000,eviction=(threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),checkpoint=(wait=60,log_size=2GB),statistics_log=(wait=0),
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten]
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten] ** WARNING: Access control is not enabled for the database.
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten] ** Read and write access to data and configuration is unrestricted.
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten]
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten]
2017-03-05T00:31:39.620+0530 I CONTROL [initandlisten] ** WARNING: soft rlimits too low. Number of files is 256, should be at least 1000
2017-03-05T00:31:39.643+0530 I FTDC [initandlisten] Initializing full-time diagnostic data capture with directory '/data/db/diagnostic.data'
2017-03-05T00:31:39.643+0530 I NETWORK [thread1] waiting for connections on port 27017
2017-03-05T00:31:40.008+0530 I FTDC [ftdc] Unclean full-time diagnostic data capture shutdown detected, found interim file, some metrics may have been lost. OK
2017-03-05T00:32:03.832+0530 I NETWORK [thread1] connection accepted from 127.0.0.1:49806 #1 (1 connection now open)
2017-03-05T00:32:03.833+0530 I NETWORK [conn1] received client metadata from 127.0.0.1:49806 conn1: { application: { name: "MongoDB Shell" }, driver: { name: "MongoDB Internal Client", version: "3.4.2" }, os: { type: "Darwin", name: "Mac OS X", architecture: "x86_64", version: "16.4.0" } }
2017-03-05T00:32:08.376+0530 I - [conn1] end connection 127.0.0.1:49806 (1 connection now open)
Depois disso, abra outro terminal e digite 'mongo'.
A seguir está a saída que você pode ver:
$ mongo
MongoDB shell version v3.4.2
connecting to: mongodb://127.0.0.1:27017
MongoDB server version: 3.4.2
Seu problema foi resolvido :)