With Defiance's transition into Free to Play for PC they've eliminated the ark hunter code bonuses that this guide was used for.

In other words, this guide is now useless.

Official announcement:
http://community.defiance.com/en/2014/06/02/last-chance-to-lock-in-pre-free-to-play-rewards/

First go to http://www.defiance.com/en/game/ and log in with your game account.

Then click the "My Ego" link and you will be presented with a page to enter Arkfall Codes to earn in game rewards, you need a total of 120 codes to unlock all the rewards.

In a new window go to http://orcz.com/Defiance:_Arkfall_Codes
This page lists 152 codes (you only need to use 120 of them).

Simply put the two pages side by side and begin copy/pasting codes. Double clicking a code will automatically highlight it for most users, making the process a bit less tedious.

To get your rewards once they are unlocked simply open the Defiance store in game and click the "Claim Items" button in the lower left.

These rewards are given to EVERY character you make on your account, so you don't have to worry about deciding which character should get which reward.

You will, however, need to go to the in game Store on each character in order to claim them for that character.

Code Input Issues:
At times the Defiance Website is known to lag out.
When I first did this I got to 110 and the website started telling me codes were invalid. I refreshed the page and it showed up saying I had 0/120 codes.

It turned out to be an issue with the site itself not connecting properly to the servers. Half an hour later I tried again, and it showed me at my 110/120. I then used the codes that were previously said to be invalid and they worked fine letting me get to 120.

So if you encounter issues inputting codes try back again later.

Alternate Unlocking:
User Acentric commented that if you are logged in on their site and contact them via the Live Chat they can manually initiate all the codes for you from their end. I have not personally verified this but it's worth a shot if you are unable to have it register the codes properly.

Alternate Arkfall Code sources:
If for some reason the main list is down here are some other Arkfall Code Lists.

http://en.defiance-wiki.com/wiki/Defiance.com -- 120 codes
http://www.arkfalls.com/ark-fall-codes/ -- 151 codes

A quick google search for "Arkfall Code List" will also produce several more sites as well.
http://lmgtfy.com/?q=arkfall+code+list -- Let me google that for you.