Click "go advanced" in the quick reply box. Start typing your message... in the section below you will see a "manage attachments" button. Click that, and another window will pop up. Click the browse buttons to locate the files on your hard drive. You can attach up to 3 items. Once you've chosen your three files, click the upload button. Wait a while to allow the files to upload. The page will change and you'll get a confirmation that they uploaded successfully. Then you can close that window, to come back to your reply screen. Finish up your message and post. Easy, right?