I'm not sure what options you will see exactly but if you go into 'Segments' you can create a segment of those users by selecting the specific app at the top of the filters. In this case, we have selected only iOS users.
That's exactly it! DavidO to the rescue as always.
Shelly It sounds like you may be on the "compose" tab where you decide whether to create a mobile or web IAM. As David mentioned, the iOS segmenting happens on the "Audience" page 3.