Multiple currency require php v5.6.x and intl extension. Please make sure about intl extension installed on your php settings otherwise you will receive blue screen or empty screen error.

Currency layer is a paid service and you have to signup that website for receive instant exchange rates.

Please check below instruction image;

  1. Enable this feature.
  2. Define your main currency by using link. Warning: This currency must be same with your custom field currency.
  3. Create an account on Currencylayer
  4. Add your convertible currencies like example.