1. where? I don't understand the question.
2. you can't add new fields. that would require custom coding work.

If you want to add a column like the amount outstanding (see the image), you will need to edit the file \components\com_invoices\views\invoices\tmpl\default.php
add this at about line 37 -->
and add this in one line at about line 78 -> items[$i]->total - $this->items[$i]->total_paid), $this->currencies[$this->items[$i]->currency_id]); ?>
By doing so your outstanding amount would be the invoice amount less the amount of payments paid.
I will also post in the other posts, just search, for calculating the outstanding amount irrespective of tentative payments set, like normal invoice apps work.
I hope this helps :-)