Keychain access stores the main certificates.

The Xcode Organizer stored provisioning profiles which rely on the certificates in keychain access.

If a provisioning profile has a message like "valid signign identity not found" then you don't have a valid certificate for it in Keychain Access.  Remember that you must have the private key for certificates in Keychain Access so you can't just download certificates created by someone else or on another computer – you need the private key the certificate was created with or you need to re-create the certificate using your own private key (which you can then save and share with others if working in a group.  If re-creating you then just need to re-create the provisionign profiles using the certifiate also with the new certificate.

UK Electronic Designer