It appears that John McCain doesn't know how to use a computer (or at least doesn't use one).
Should the US President know how to use a computer?
Well, there have been at least 40 US Presidents who haven't been able to use a computer. So the answer has got to be "no". They have enough staff to avoid it.
But in 2009, one would expect the US President to be fairly computer literate. I would expect the US public to expect their President to at least be able to tap out the odd email and Google something.
As Taegan Goddard observes:
The admission that McCain doesn't use a computer is startling. In 1992, President George H.W. Bush took a beating for not knowing the price of milk as the country slipped into a recession. It's hard to imagine anyone in 2008 electing a president who doesn't know much about a fundamental building block of the modern economy.