Well, you click the "Manage Attachements" button below the textbox in the "Reply to Thread" screen.
Click "Browse" and find the replay you're looking for in the "Replay"-folder in your toribash directory.
Click upload. You can add more than one replay, if you'd like.
Then exit the small window, and submit your post.
EDIT: Don't worry about opening a new thread in support. The mods are likely to move this over eventually.