don´t know anything about SPF, never give it a chance before. Bur I advice you to take care that on many systems, postfix runs in a chrooted enviroment. (e.g. in debian)
Also in debian you need to install postfix-policyd-spf-perl
I think on another Distribution there is also an extra package wich contains the script you are searching, because SPF is far from being a thing which should be in the main package of postfix, because its not yet spreaded so widely.
according to the site you mentioned, postfix awaits a socket:
28 check_policy_service unix:private/spfpolicy
Run the mentioned script or any other (there are more available) an tell the script to listen to the specified socket.
The socket must be created in the folder private of the chrooted enviroment, if your distribution uses chroot for postfix.
If the spf deamon script creates this socket in a place outside of your chrooted enviroment, postfix will not be able to reach it.
So take care.
Take a look in master.cf there you can see if postfix uses chroot. There must be a no in that line, if there is a - than its chrooted.
If your SPF deamon hast the capability to listen on a port, it may be safer to use this, because it doesnt matter if you are chrooted or not.