Try the following it will work
i.Open the terminal
ii. Enter mongodump --collection collectionname --db dbname (Don't go inside mongo shell);
iii.If default port is different(other than 27017) then go for the following command
mongodump --host mongodb1.example.net --port 37017 --username user --password pass --out /opt/backup/mongodump-2011-10-24