Perform the following steps to create an iOS Distribution Certificate:
Create an iOS Distribution certificate from the Apple Developer site at http://developer.apple.com.
Sign in with your Developer account and click on Certificates, Identifiers and Profiles, and on the next screen, click Certificates.
Click on the Add (+) button on the certificates page and select the App Store and Ad Hoc option, and then click Continue.
Before proceeding to the next step, use the following steps to create a new certificate on your Mac using the Keychain Access application:
Open this Keychain Access application.
On the Keychain Access menu, select Certificate Assistant, and then click Request a Certificate From a Certificate Authority.
The Certificate Information window appears, where you can enter your Developer account email address and the common name, which is typically the name of your organization.
Select Save to Disk to save the certificate as a CSR file on your computer, and press Continue.
In the Certificates dialog box on the Apple Developer account page, click Continue to move to the Generate tab.
Upload the CSR file that you just created.
Click Generate to create your iOS Distribution certificate.
Download the generated file, which is an iOS distribution (.cer) file, and save it to your Mac system.
Double-click the .cer file to install the certificate in Keychain Access.
Note: Save a backup copy of the certificate in a secure location.