missing private key in the distribution certificate on keychain

As long as you still have access to the mac which was used to generate the original distribution certificate it's very simple.

Just use that mac's Keychain Access application to export both the certificate and the private key. Select both using shift or command and right click to export to a .p12 file.

Attached a screenshot to make it very clear.

On your mac, import that .p12 file and you are good to go (just make sure you have a valid provisioning profile).

Ahh this is a common issue, The solution is simple:

Who ever created the developer credentials originally needs to go to the keychain on their computer and right click on the key(s) for private and public and export the key to a file. Then you just download that file on your computer and open it, and it will be added to your keychain.

You need to have both the private key (.pem file) and the certificate for your provisioning profiles.