Hi.
In Mac OS X and Linux, you can use the default Terminal window to establish an SSH communication, because SSH comes in all Unix-based OSes.
If you’re trying to connect with a Raspberry Pi, follow the next steps:
 
1) Boot up your Raspberry Pi.
2) Open a new Terminal window in MAC OS.
3) Type the following command:

$ sudo ssh pi@raspberrypi

or:

$ sudo ssh pi@raspberrypi.local

Note: if you defined a different hostname and username rather than raspberrypi and pi, you should use those instead.
4) Enter your computer password (so you can run a sudo command), and type yes.
5) When you’re asked to type a password for your Raspberry Pi type the password you’ve set previously, press Enter/Return.
When you connect your computer to your Raspberry Pi for the first time, you’re prompted by a message warning you that you’re attempting to establish a connection with an unknown host. Simply click OK to proceed
Your Terminal window should look like the Figure below:

Let me know if this helps.
Regards,
Sara