As I understand it, this requires a special airbag deactivation bar be installed under the seat and the system deactivated using a PIWIS or PST2 diagnostic tool which pretty much only the dealer has, the PN# is: 986.025.01 .
I've heard that Porsche does not sell this bar separately, but bundles it with the Porsche Child Seat which runs $500-$700.
Sometimes you can find the bar only on ebay and they susally go for around $100.
You can check this link for more info:
http://www.gagme.com/greg/Boxster/hacks/babyseat.php