Ios push notifications certificate has no associated key

Question

I want to create an iOS application that uses push notifications. Notifications will be triggered by the PHP web service. To do this, I needed to create an iOS push service certificate in the iOS provisioning profile (which must be added to the web service to work). I downloaded the certificate and added it to access the keychain.

The documentation at https://developer.apple.com/library/ios/#documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/ProvisioningDevelopment/ProvisioningDevelopment.html says:

• Open the Keychain Access utility and select the My Certificates category in the left pane.
• Locate the certificate you want to install and expand its contents. You will see both the certificate and the private key.

I did just that, and I found the certificate in the right place, but there is no key associated with it. Where am I probably mistaken, why is there no key and how can I fix this problem?

thanks