Edit the RTD_functions.php file, and change the $RTD_API_USERNAME, $RTD_API_PASSWORD and $RTD_API_SIGNATURE values to match your RTD API credentials.
Such values can be obtained in your RTD admin panle by clicking the Manage API Credentials link on the left area.
Upload all files into some folder of your host (for example, http://www.yourdomain.com/RTD_API/)
Test the API: Visit with your browser the RTD_get_product_ids.php file. Should be something like:
http://www.yourdomain.com/RTD_API/RTD_get_product_ids.php
If all is ok, you should see an exploded array, with all of your categories and products. Congratulations, your API installation is working!
