Warning: This tip is only for Gmail users outside US.
Google says that the latest Gmail feature that allows you to make phone calls is only available in the US, but that's not true. If you set Gmail's interface language to English (US), you should see the "Call phone" option in the chat roster. You can also buy credit using Google Checkout.
If you're not in the US and you want to add credit to your account, make sure that you DON'T CLICK the "Upgrade your account" button from Google Voice. This feature is only useful if you are in the US, since it lets you get a Google Voice number. Unfortunately, if you click on that button and you're not in the US, you'll no longer be able to buy credit.
Here's what you'll see after clicking on "Upgrade your account":
I don't think there's a way to downgrade your account, so the best thing to do is to avoid clicking that button. Google could easily solve this problem by hiding the button for non-US users or by allowing users to stop the upgrade.
Update. You can cancel the upgrade if you use a US proxy (try UltraSurf if you use Windows).
Update 2. A Google spokesperson said: "We're looking into this and will keep you updated."