I assume you are using the font properties menu that you get when you right click on some selected text. Go to the character spacing tab. Next to the the box that you check to request kerning, you will note a box where you specify the the point size at which Word 2007 starts to apply kerning. Did you choose a size small enough to include the point size you are using? If this does not work are you certain that your Simoncini Garamond from Adobe has kerning?
Don

I know how to activate kerning in Word 2007... but at this point I'm not sure Simoncini Garamond from Adobe has kerning: how can I see this?
http://store1.adobe.com/cfusion/store/html/index.cfm?store=OLS-US&event=...

Is the Simoncini Garamond from Linotype different?
http://www.myfonts.com/fonts/linotype/simoncini-garamond/simoncini-garam...

Another question: why is kerning off by default in Word 2007? Is there any drawback of kerning?

> but at this point I'm not sure Simoncini Garamond from Adobe has kerning

It does. Not vast amounts, but it has kerning. You can look at some specific combinations that usually involve substantial kerning to check, such as:
To Te
LTAVA

> Is the Simoncini Garamond from Linotype different?

It may be. Adobe originally supplied all their versions of fonts they licensed from Linotype back to Linotype, both originally in Type 1 and later in OpenType conversion. But Linotype has since updated some/many (all?) of them. You would have to ask Linotype. Perhaps one of their folks will comment.