Magento has multi currency capability built in. When it is configured and enabled, your customers will see a currency drop, like in the image below:
Where the currency drop down is displayed is determined by your template.
However, you won’t see the currency select control if you haven’t actually imported your currency exchange rates. You import your currency exchange rates within the Magento administration ui, under System/Manage Currency Rates. There you can use the “Webservicex” webservice, see below:
Only after these rates have been imported and saved will your customers be able to see the select currency control.
So, if you’ve setup multi-currency within your shop but the currency drop down is not visible on the front end, first make sure that you have actually imported and saved your currency rates.