My siddur says:
מן ר”ח אלול עד אחר שמיני עצרת בערב ובבוקר אומרים זה ויש נוהגין לאמרו קודם ח”ק שאחר שמו”ע From Rosh Chodesh Elul until after Shmini Atzeres, in the evening and in the morning, we say this. Some have the custom to say it before the Chatzi Kaddish after Shmone Esre.
Although the common custom in Nusach Sefard is to say this at Shachris and Mincha, it seems to me that by "evening and morning" the siddur is saying to follow the Ashkenaz custom of saying at Maariv and Shachris. Could it possibly be understood to be referring to Mincha rather than Maariv? Secondly, is it safe to assume that it is placed after both Shmone Esre and Tachanun, directly preceding Chatzi Kaddish?