From the Ezras Torah Luach about tonight:
At Maariv, we begin to include "ותן טל ומטר" into the Shemonah Esrei. If one became aware, after he had completed the Bracha מברך השנים, that he had omitted "ותן טל ומטר" he should wait to insert it right before "כי אתה שומע" of שמע קולנו. If he had already completed the Bracha שומע תפלה, he may insert it before saying "רצה". If he had already begun "רצה" he must return to the Bracha "ברך עלינו", which is the proper place for "ותן טל ומטר". If he had already completed the Shemonah Esrei and stepped backward, then he must repeat the entire Shemonah Esrei. In any situation in which a person must repeat the entire Shemonah Esrei, he may fulfill his obligation by listening to every word of the Chazzan's Repetition from begining to end, with the intention of thus fulfilling his obligation.