Hi windchest,
Uploading is easy ... click on the "Go Advanced" button below the reply box, then click on "Manage Attachments". Find the file on your system using the 'browse' feature or by entering an url.
Another window opens at which time you click on "upload". After the upload is complete (please heed the max file sizes for uploading) you simply click on "Submit Reply" and VOILA ... your post along with the attachment is posted.