What works for me dealing with Push Notification certifies has been:
- Open the certificate:
open my_filename.cer
and click “View Certificates” to see the certificate’s name - Go to Applications -> Utilities -> Keychain Access
- Ensure you have selected the “login” keychain, not the “system” one (thanks to @Matt Flettcher )
- Go to “Certificates”
- Look for the desired certificate
- Ensure that you can expand it and see under it the original “.certSigningRequest” used to generate the certificate
- At this moment you should be able to export it as “.p12”