The paragraph of "Anenu" is added in the "Shome’a Tefila" section of all the Amida prayers recited on Tisha B’Ab, just as on other fast days. Additionally, during Minha we insert the Beracha of "Nahem" in the section of "Tishkon Be’toch Yerushalayim." This Beracha is recited only once a year, at Minha on Tisha B’Ab.
As it is recited only once a year, it is very common for people to forget to recite this prayer. If one forgot to recite it and realized his mistake only after he concluded the Beracha of "Boneh Yerushalayim," then he should recite it in the Beracha of "Reseh," at the point where "Ya’aleh Ve’yabo" is recited on Rosh Hodesh and holidays. If one mistakenly recited "Nahem" in "Shome’a Tefila," together with "Anenu," he has fulfilled his obligation and does not then have to recite it during "Reseh."
If one did not realize his mistake in time to recite "Nahem" in "Reseh," then he should recite it at the end of the Amida, in "Elokai Nesor." If he did not realize his mistake until he completed the Amida, he does not have to repeat the Amida.