It's really easy Cindy, here's how to do it.
Go to whatever it is you want to link. Go up to the "Address" bar and click on it. Now it will be blue. Right click and click on copy. Then go back to FK and in "Post Reply" at the bottom put your cursor in the box that says "Link URL" and right click and click on paste. When you submit it it will show as a link. Easy!