Your blog RSS URL is the URL which users use to subsribe to your RSS feed. You can generally find this by going to your blog and finding your Subscribe Via RSS link. Your Blog RSS URL is the link that comes up in the brower after you click the Subscribe to feed link.
As a general rule, blogs hosted on WordPress are typically formatted like http://myblogdomain.com/feed. Those hosted on Blogger are typically formated like http://myblogdomain.com/feeds/posts/default. Finally, those using Feedburner are typically formatted like http://feeds.feedburner.com/MyBlogName.
If you still need assistance, please feel free to contact us. We can help you find it.