The URL for any given page is in the "address" field of your browser when you are on that page.
Highlight it by clicking on it, copy it using the copy command under the edit menu (or, more easily, hit "control-c"). The click on the URL link in the Instant UBB Code menu under the box you type your post in. When the new tiny window appears at the top of the page, make sure your cursor is in it and do "paste" from the edit menu (or "control-v"). This pastes the URL address into the tiny box. Hit "enter." A second tiny window appears and in that one you type what words or "title" you would like to show as the link... it might be "my story," "history," or if it's an amazon address, the title of the book. It takes much longer to write it out than it does to do it!
