How to Setup a Dual Currency Magento Shop
Login as admin and select System->Configuration, then select ’Currency Setup’ from GENERAL section,
then select Base currency, Default display currency, allowed currencies (press Control Key and Click the needed currencies) and ’Save Config’
Now come to "System->Manage Currencies’ menu
Click in Import currencies to load the exchange rate(s).
You are done. Visit the frontend and reload (you may need to refresh your cache) to see the drop down added to your product pages.