When you set the stalk to variable (all the way down), you can adjust the wipers from 4-30 second wipe intervals with the dial. As far as I know Boxsters never came with the dial from the factory, and if it isn't doing anything you might pop it out and see if it is hooked up, maybe a previous owner just put the dial in to get rid of the dummy cover. I figured out how to add the dial and to be honest rarely use it, but I like not having any dummy button covers on my dash.
I have more information on my website if you want to know how to hookup the dial:
http://www.bluboxster.com/Mods/Variable_Intermittent_Wiper_Hack_-_Primer.html
http://www.bluboxster.com/Mods/The_Porsche_Way.html