To enable an International shipping option click on Selling » Prices, Shipping, Tax » Shipping.
In the upper right corner of the resulting page, click on the Add or Remove Shipping Options link, put a checkmark in the box labeled "International", and then change the label in the corresponding textbox if you'd like the option to show up as something different on your storefront. Then, click Save.
Once the new option has been added, click the Edit button next to each of the displayed products and/or shipping tiers to define the International shipping rate for that item.
Note: When enabled, the international option is automatically selected for orders with a shipping address outside of the country associated with your account.
Disabling this option DOES NOT disable the ability for your international customers to place their orders; it simply removes the shipping option from being displayed during checkout. (Your international clients can still check out using any of your other shipping options.)
Similarly to the other options, this shipping option can be renamed to anything that you'd like. The most common name is simply "International Shipping."