This is a very simple thing to do, but it constantly gets asked so here is a quick tutorial of how to enable the Quick Reply.
First go to the Admin Panel then and select current theme



Now at the top there is a tab for Reset Options



Now go to the theme you are using and select "Reset all members' current options for this theme"

In this area there are a few other helpful settings:



And to enable Quick Reply all you do is go to "Use quick reply on topic display" and from the dropdown choose change, and finally "show, on by default"

It is that easy!

I also recommend turning on:
Show a popup when you receive a new personal message?
Show most recent personal messages at the top
Return to topics after posting by default.

This tutorial is courtesy of Lord Anubis.