The first thing to do is get your CSV file from ACX. To do this, log in in your ACX account.
Then go to View Sales Dashboard:
Then click the promo codes tab:
Click the Get Promo Codes button beside the book title:
Once the resulting pages loads, Click the Export CSV link to the right:
DO NOT EDIT THE CSV FILE! IF YOU DO, IT WILL NOT WORK!
Now, head on over to FreeAudiobookCodes.com and log in to your account. Once you do, you'll be taken directly to the My Account page where you will need to buy a package:
Once you do that, go back to My Account and click the Upload Book button:
Fill out all the information in the resulting form and click the Next - Upload Promo Code csv button. Select the csv file you downloaded from ACX and hit submit. That's it! Your book is now on the main page and ready to go!