I missed a step, sorry. "Go Advanced" will load a new page, not a pop-up box. So you'll have an expanded area in which to reply, and lots of extra features under the area where you type text. The pop-up box appears after you click the "Manage Attachments" button under the "Additional Options" banner. If you still don't get a pop-up box, either your browser or a security program may be blocking pop-ups.