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.