Spectacu.la Discussion adds threaded commenting with live AJAX comments to almost any WordPress Theme.
Unfortunately it's not possible for us to check this code against every theme out there so from time to time you may run into some incompatibilities. It's possible that some CSS in your theme's style.css is causing issues. You could try stripping out all commenting related CSS from it and see if that makes any difference. If it's not that then it might be that your theme isn't calling comments_template() to add the comments code, if that's the case then you're out of luck unless you know where to look in the theme code or you can get your theme developer to change their theme.