If you want to add a comment to a thread, then just click on
at the bottom. If you want to reply to a specific comment then you can click on
and add your response. That is how I included your question in this post. Finally, if you post something and aren't happy with it (spelling errors, etc), then you can click on
to go back and fix it.
That and plenty more info can be found in the FAQs (Frequently Asked Questions). There is a section for general forum help and other section for INGO. There is a lot of good info there.