You Need The Private Key!!!

You can't just download a certificate someone else created or that you created on another computer.  If you don't have the private key the certificate was created imported into Keychain Access with it won't work!  Helpfully, keychain access wont tell you this but when you click on the downloaded certificate Keychain Access will open and show you "My Certificates" but the downloaded certicificate just won't appear. You can do the following to resolve:

Find the original private key that was used to create this certificate and import it to keychain.

Revoke the certificate and create a new one. Don't worry, it will not affect your ability to deploy on the App Store. Just remember to recreate the distribution provisioning profiles as well.

UK Electronic Designer