I promised you two mistakes. Here is the other one. I did NOT see a buy button. If you do not see the button, you have the graphics turned off. I think I diabled images when I had a problem with the "New York Times" locking up the experimental browser. Go to Menu/Browser Settings to enable graphics and then you will see the "Buy" button and you are on your way!
Please note that if you are a US resident, you may only use the US store - no matter where you might be traveling- and if you are a UK resident, you must use the UK store.
As a US resident and an Anglophile, I would love to register my Kindle to the UK store, but that would only be possible if I had a UK credit card sent to a UK address. My UK bank account is not sufficient to meet the "residency" requirement.
If you move to another country, you may change which store your Kindle is registered too - but your Kindle may only be registered to use one store.