I've just tested in my game and it's opening fine, I'm also not seeing any other reports of this and if Gold couldn't be purchased there would be mass hysteria. 
Have you tried another browser? Have you turned off your pop-up blocker? 
Which window exactly is it that won't open? Is it the one that shows you the gold purchase prices in game or the one that opens after you click buy that allows you to choose a payment method?