Google Exchange rates are provided by http://itechforest.com - pay for one time and get unlimited requests with unlimited stores access.
Just drop the folder it into \Plugins directory on your server.
Limited time offer for the Buyer on first come serve bases! Download this theme for only 150$.
Go to your plugins screens
Find the Google finance exchange rate provider
Click Install In your Currencies screen, select the Google finance exchange rate provider and click Save
Click Get Live Rates To Update your Exchange rates from google.
Full docs and screenshots: http://docs.nopcommerce.com/display/nc/Currencies