When you hit "Reply" and are taken to the "Post reply" screen, there will be a link below the reply box titled "Attachments and other options". Click it, and it will expand to show a box titled "Attach" with a "Choose File" button. From there you just click that and find the file on your computer you want to upload.
Please note the upload restrictions below the box:
"Allowed file types: doc, gif, jpg, mpg, pdf, png, txt, zip
Restrictions: 4 per post, maximum total size 4096KB, maximum individual size 4096KB"