To connect applications to your Prestashop store you will need to enable webservices for your store, and you will have to create a webservice key. You can do these steps through the user interface or programatically.
Login to your Prestashop back office. In the bottom left hand corner click on the "Advanced Parameters" and then select "Webservice". Change "Enable PrestaShop's webservice" to Yes.
Now that webservices are available, click the "Add new webservice key" button. On the new menu, you will want to click "Generate" to create your new key, then make sure the "Enable webservice key" is set to Yes. When selecting Permissions, put a check-mark in "View (GET)" on all the resources. You will have to scroll to get them all. When you are finished click "Save".
Open up greenback.com, login, and visit https://www.greenback.com/app/connects/prestashop -- you'll then be prompted for your "Store URL" and "Webservice Key". Enter your stores homepage URL and use the webservice key generated in step two. Click "Connect" and your store should be successfully connected.