Problems installing .cer files

I was having provisional profile problems installing my development iPhone app, so I tried erasing all my certificates and starting again. Now when I double click AppleWWDRCA.cer, developer_identity.cer, or distribution_identity.cer I get this error:

The “System Roots” keychain cannot be modified.
To change whether a root certificate is trusted, open it in Keychain Access and modify its Trust Settings. New root certificates should be added to the login keychain for the current user, or to the System keychain if they are to be shared by all users of this machine.

I can't find any help from Google. Does anyone know the problem?