xdass You need to access SSH, If you don't just do it on your local computer.
Windows
You need to have Composer installed. I assue you already have that since you installed flarum.
open up your CMD point it to your flarum installation root and write
composer require xengine/flarum-ext-markdown-editor
OSX
It's the same as windows use Terminal instead of CMD ?
Finally
Go to your vendor directory and copy folder named XEngine to your FTP's vendor directory. I think you are good to go man!