Pandora blocks users outside the US by looking at each visitor’s IP address, a string of numbers which identifies an individual computer. The site then blocks access if the address is registered as being outside the US.
You can get round this problem by using a service which hides your IP address. There are a variety of services which do this, some of them for a fee.
One free service is Hotspot Shield, which is primarily designed to make it harder for snoopers to intercept your Wi-Fi signal (as they won’t be able to find it through an IP address). The main drawbacks to this service are that it displays an advert at the top of each page while you use the internet, and that it can slow connections to the point that watching streaming video is difficult or impossible. However, it is easy enough to start and stop that you can simply use it when you want to visit Pandora.