Do you know, the new AdSense Interface helps you understand your AdSense performance based on where your users are using countries report? To report by country, simply navigate to the Performance reports tab. In the navigation sidebar, click on the "Countries" report. This report will show your site's earnings by visitor country.
Please note that the countries report won't contain historical earnings data prior to when this feature is made available in your account. You can view earlier click and impression data by countries, but historical earnings data won't be visible.
Try it now! Navigate to the Performance reports tab, and select the countries report.
[Source: AdSense blog]