Software Center not Signing Me In

Ubuntu 12.04
Ubuntu Software Center

When I choose a program - in this case 'Puzzle Moppet,' &' Blocks on Ice,' then click on 'Buy', even tho it's free, it gives a white screen with the words 'connecting to payment service.'

Then pops up with a box that says 'Failure in the purchase process. Sorry, something went wrong. Your payment has been canceled.'
 It has been doing this for some time with other attempts at buying free items.

In the past before, I've been sometimes successful, & sometimes not. But the times it wouldn't work, it eventually resolved itself, with no idea how or why it did.

What is the deal, here?

Also, how do I sign in to the purchase center direct, without having to download a game, so perhaps the error won't occur?

Thank You