1. Quick reply box
2. click "Go Advanced" button
3. Scroll down to "Additional Options" section
4. Attached Files > Manage Attachments
5. Choose File
6. Go into your replay folder
(personally, mine is in (C:)>Games>Toribash-4.62>replays )
6. Select desired replay
7. Upload
EDIT: Fuckin' loved that, Warrior