I will be following the official documentation here but they take some things for granted, many users could get frustrated and confused that's why I created this gist.
I have downloaded the mongo files in my root user directory for example: /Users/israel/mongodb therefore if you want to do the same first enter cd in your terminal.
then enter pwd command in your terminal and you should be able to se something similar to /Users/<yourmacuser>
If the previous is correct you may want to proceed and open the terminal then download the tar: