You have to patch both the kernel and ppp with pppt/pppe.
I found it easiest to download the latest stable kernel from kernel.org and use the patches from there as well. Download ppp source and pppe patch and compile it.
Then recompile the kernel and activate the modules:
modprobe ppp_mppe_mppc
and: modprobe ppp_generic, modprobe ppp_async, modprobe ppp_deflate
here are some webpages that walk you through:
http://pptpclient.sourceforge.net/
http://it.geocities.com/puccio_13/fantoft-linux.html