I'm trying to export the APNs certificate as a .p12 file.
but, after I double click on the .cer file, I can't find it under "My Certificates", and I'm not allowed to export it as .p12 file (but .cer, .pem, p7b).
why? I created this certificate on the same computer.
Issue seems to be with you private key. If private key associated with certificate is missing then it will not get displayed in keychain under "My certificates" sections. You can still see under all certificates. Create a new certificate with new certificate signing request file and try to install it on mac. This may solve your problem.