If you'd like us to publish the app in your developer's accounts, in addition to access credentials we'll need your iOS distribution certificate that we'll be using for signing the app.
A .p12 file is a specially-formatted and encrypted file that contains your distribution certificate. It's embedded by the Mag+ Publishing portal when building your app. iTunes Connect checks for this file when you submit an app and will only accept the app if it contains a .p12 file that matches what you have configured in your iTunes Connect account.
1. On your Mac, launch Keychain Access, select the certificate entry and right-click on it to select "Export."
All certificates you have installed will be in the "login" keychain (Label 1) under the category "My Certificates" (Label 2).