you will have to replace that name with the actual filename including path if it is in a different directory

as in the description firmware images are:
http://tablets-dev.nokia.com/nokia_N900.php
and for uk(i don't know what is special about uk but..)
http://wiki.maemo.org/Maemo_UK_variant_firmware
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otherwise

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you find all kernel in the repository
http://repository.maemo.org/

e.g. the standard kernel:
http://repository.maemo.org/pool/fre...free/k/kernel/
if you like me to lazy to search for the fiasko(it is somwhere.. ) you do:
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ar kernelwhatever.deb
chdir kernelwhatever
tar xzf data.tar.gz
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OR you just use some grafical unpack program like winrar or whatever you have that can unpack ar and tar(same copression type) to get the fiasko out of the deb. (this is click-click.. done)