We insert the tefillah of nachem into the berachah “v’leey’rushalayim ir’chah” at Minchah of Tisha B’Av. It is said even if one is not fasting. If one forgot to say it in its proper place, it is said in the place of ya’aleh v’yavo, before “v’sechezenah…” – and NO special chasimah is said. If one forgets to say it there as well, it is said before (the second) yiheyu l’ratzon, without any chasimah.
If one must eat on the fast of Tish’a B’Av, nachem is said in bentching in the ya’aleh v’yavo slot (and no special chasimah).