Social Bookmarks Backlinks

Im just trying to learn about building backlinks from different places. Places like Social Bookmarks forums is what i dont understand. After you sign up i still dont see where you can add in backlinks from. In the users posts, when i hover my curser over any hypertext, it shows the forums URL in the taskbar with says to me there is no link juice there. But i keep seeing people talk about getting links from these sites but i dont see how?