Before you install make sure you agree Xcode license.. for that just goto terminal and run the following command
sudo xcodebuild -license acceptNow open terminal and run the following code to install brew.
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"what is brew? http://brew.sh Homebrew installs the stuff you need that Apple didn’t. You can install robomongo app using brew..you can even install mongodb using brew like that