I was wondering too if it works with 3.5.3 as I can't seem to get it right. But I'm not an expert and it could very well be my own fault.
Anyway, I followed your instructions and got a blank page when I went to .../forums/modapp.php
I tried d/ling the .zip file again [as suggested by someone], re-uploaded everything and also created a new template called 'modapp' in my parent style with the code you provided in the .zip file, but I still get that blank page. Can anyone help me please - and in dummy language as I'm really not that familiar with hacks.