Whenever I find I have learned how to do something which involves using computers or using the internet it changes. This invariably happens when a particular software application is updated.
I wanted to top up my account on iTunes by adding money to my account which I had set up, some years ago, on the computer. I spent ages trying do it. Ho hum ... everything had changed... but I figured it out eventually.
This is what needs to be done to put money into an existing iTunes account. For starters you cannot do it directly from your computer.
You have to purchase a "Gift card" from your local outlet (I assume in the desert these are in handy locations) and do it that way.
To redeem it:
 Use main iTunes home page.
 Top right near Search bar is Name (of your ACCOUNT) typed in plain font, lower case.
 Clicking on this there are 4 items: Wish List, Recently Purchased, Redeem and Account Info. Click on Redeem.
 At this point in time the way to operate the card is to either scan (which worked) or type in the code number on the back of the card.
 My problem was that I was trying to put money into the account using the part of the site called 'Account Info', as was the case previously. There is no instruction here to go to a different part of the menu (namely, up the way) where it says REDEEM.
Photo: Mac OSX Leopard and iTunes card (mine).