Please make sure you have "release.2016.14" or later of the AEM Mobile Signing Tool. To download the latest signing tool, you may follow directions here: Signing iOS and Android apps for AEM Mobile.
Then, use the AEM Mobile Signing Tool to sign the application you downloaded from the Apps section of the On-Demand Portal. Be sure that the Certificate you choose matches the Mobile Provision you choose. This relationship was established when you generated the mobile provision in the Certificates, Identifiers & Profiles page on developer.apple.com.