Authorize.net Integration

I'm using the MarketPress eCommerce plugin. I've never been able to use Authorize.net because I keep getting the following error: "There was a problem finalizing your purchase. The configuration with processor is invalid. Call Merchant Service Provider. Please go back and try again." I have the right login ID and key code for Authorize.net. I also have a validated SSL certificate with the applicable links in the filter to secure the site. Why is there a configuration issue? Is there something I'm not seeing?