Okay, tripped slightly.
This is what I've done:
1) In showthread.php, added the necessary code.
2) Created a new template, with the name, "showthread_replybox" and added the same code in there.
3) Went to Show Thread Templates > Expand > showthread and added $replybox right after $pagenav.
4) Then I uploaded showthread.php.
The result is zilch, nada, zip. I know I've screwed up, just tell me where.