You probably use a very old version of WordPress or PHP version below 4.1.0, otherwise your website must support DOMDocument:
Feedweb (like the most professional WordPress plugins) requires PHP 5.0 or higher.
To find out the PHP installation you have, please run the following url:
If you don't get phpinfo, you can install Wappalyzer extension to your Chrome browser and retrieve your website information from Wappalyzer.
Please send us any question you have to firstname.lastname@example.org and we will try to help you immediately.