When you are uploading your certificates into Carnival, they must be in .p12 or .pem format. From your Keychain Access, right click on your certificates, and select the Export option. You should receive an option with .p12 already selected.
If the option for you is not able to be selected because it is greyed out, like in the screenshot:
Make sure you are in the My Certificates or Certificates category in Keychain Access.