This is a connectivity error not uncommon with certain networks every now and then. It usually goes away on its own. But you can force stop the 403 error and download apps successfully by following these simple steps;

Go to Settings> Wireless and Network settings> Mobile Networks> Access Point Names.

Now open whichever access point you use to browse on your phone (usually the one which is currently selected). In it, if there are a set of numbers in the PROXY, remove them and click Ok. Now, go download apps:-)