One of the things to consider when adding RSS feeds is that, when you want to add an URL it is important to add a "feed" at the end of the URL, for ex: would be feed/ 

Keep in mind, only URL's with RSS feeds can be used, you might find that some of URL's do not have any RSS feeds to import and thus Content Artemis won't import them:

For more information, I would highly recommend reviewing the tutorials page in Content Artemis, there is a step by step video on how to import RSS feeds into your Content Artemis as well as other very useful information.