Can't install "Google Web Toolkit Developer Plugin" under Chrome

I was starting coding with GWT following the tutorial at

http://code.google.com/intl/it/webtoolkit/doc/latest/tutorial/create.html

when the tutorial tells to connect to the 'development mode code
server' the browser ask to install a plugin.
With Firefox everything works fine, but with Chrome every time I try
to connect to the server it asks to install it

http://code.google.com/intl/it/webtoolkit/doc/latest/tutorial/images/MissingPlugin.png

even if it is listed under

chrome://extensions/

I'd like to know what am I missing.

TIA

Something useful
GWT developer pulgin version: 1.0.9738
Chrome versione : 12.0.742.112

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