Cannot convert apple developer_identity.cer into .p12 format. No certificate matches private key

Question

i have following problem:

i have these files

developer_identity.cer

Team_Provisioning_Profile_.mobileprovision

In order to package adobe flex mobile application for iOS, i need to convert my .cer certificate into .p12 format. Following this tutorial on help.adobe.com i always get this problem when executing last openssl command:

"no certificate matches private key
error in pkcs12"

From what i understand i need somehow to get private key, that was used to create the certificate (do i understand this well??). How do i get the private key mykey.key if i only have .cer and .mobileprovision files mentioned above?