To use the spoiler tag, type your spoiler and then highlight it by running your mouse cursor over it.
Once all of your spoiler text is highlighted, click the black box in the upper right hand corner of your post. In the window that pops up, type "Spoiler here" or something along those lines to indicate a spoiler. Click "OK" and the click "submit reply" for your post.
Your spoiler should now only show when someone clicks on your spoiler tag in your post.