Nacheim:[1]

The blessing of Nacheim is recited during Mincha, in the blessing of Boneh Yerushalayim.[2]

If one forgot to say:[3] If one forgot to say Nacheim he has nevertheless fulfilled his obligation. [If one remembered to say Nacheim after finishing the blessing of Boneh Yerushalayim, then he is to say it in the blessing of Shema Koleinu, after Aneinu[4]; or in the blessing of Avoda after Ritzei prior to the words Visechezena[5] [in same area we say Yaaleh Veyavo]; or in the blessing of Hodah prior to the words Veal Kulam[6], or in Elokaiy Netzor prior to the second Yihyu Leratzon.[7] In such a case he is not to conclude with the blessing of “Menachem Tziyon” but is rather to continue with Visechezena until the end of the blessing.[8]]

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[1] Michaber 557:1; Rama ibid only by Mincha

[2] The reason: The custom is to say Nacheim only by Mincha as at that time the Mikdash was burnt and we hence ask to be consoled. [Rama ibid] The reason it is said in Boneh Yerushalayim is because it relates to the prayer. [M”B 557:2]

[3] Michaber 557:1; 565:2; See Kaf Hachaim 557:2

[4] Taz 557:1 in his main opinion; Mateh Yehuda 557 in his second option

The reason: As it is a regular request like all others which may be said in Shema Koleinu. Nonetheless, Aneini is to be preceded being that it is more common. [Taz ibid]

Other opinions: Some Poskim rule it is not to be said in Shema Koleinu, but rather in Avoda. [See M”B 559:2] Some Poskim rule that if one said it prior to Es Tzemach David even with a blessing that it is valid and is not a Hefsk or a blessing in vain. [Moed Lekol Chaiy 10:85; Kaf Hachaim ibid] Others however rule that while it may be said after Boneh Yerushalayim, it may not be said with its closing blessing. [Rashash Taanis 30a]

[5] Taz ibid in his interpretation of Abudarham; Mateh Yehuda 557 in his main opinion; Rabbeinu Yerucham Nesiv 18:2; M”B 559:2 in his final opinion; Biur Halacha “Beboneh Yerushalayim” in name of Reah in his Chidushim on Rif and Chidushei Haritva in name of Yerushalmi that it is said in Avoda and not Hodah; Moed Lekol Chaiy 10:85

[6] M”A 557:1 and Beis Yosef 557 in name of Abudarham p. 111; Yerushalmi Brachos 4:3; Elya Raba 557; The Taz ibid negates this opinion as a misprint in the Abudaham and that really it should say Avoda, while the M”B ibid negates both options. Practically one may follow whichever opinion one chooses. [Likkutei Mahrich; Nitei Gavriel 85:18]

[7] Kaf Hachaim 557:2

[8] M”B 559:2