When you make a post, then you click the "Post Quick Reply" button right?
Well, right next to it there is a button named "Go Advanced". Click it.
Scroll down, and click the button named "Manage Attatchments".
Then you click pick/choose file.
If it doesnt automatically go to your replay folder then do this:
1. Go to "This computer"
2. Go to Local disc/hardware
3. Go to the folder named "Games"
4. Double-click the Toribash folder
5. Double-click the Replay folder
6. Pick a replay.
I hope it helped :P