When do we say Yaaleh Veyavo
Yaaleh Veyavo is said on Rosh Chodesh and certain festivals inside the Amidah. In the digital siddur it appears in Shacharit according to the day. The insert is said on days that require mentioning the occasion, such as Rosh Chodesh. In Shacharit it sits inside the Amidah, in its fixed place in the nusach. In the MySynagogue siddur you can open Shacharit and read the Amidah with the inserts for that day.
Also at Mincha?
Yes, on the relevant days also at Mincha and Maariv, according to custom.