You will need to look for the little RSS feed icon on a forum and then click that and it should give you the URL to put inside the RSS Feed Importer. Mind you this is copyright violation if you don't have explicit permission to do so. Nonetheless it's well worth doing it if you do have permission or have found a viable source. There are many good RSS feed providers such as Yahoo and many others. You just have to find them. Getting the URL is different depending on who you choose. But some sites/forums have that little icon that shows and brings you to the RSS feed page.