developer_identity.cer <= download from Apple
mykey.p12 <= Your private key
openssl x509 -in developer_identity.cer -inform DER -out developer_identity.pem -outform PEM
openssl pkcs12 -nocerts -in mykey.p12 -out mykey.pem
openssl pkcs12 -export -inkey mykey.pem -in developer_identity.pem -out iphone_dev.p12
cer文件 down下来后点击再导p12