If you are planning to use "Advanced Download Restrictions ( optional, for greater flexibility )" you will need to install a plugin called / like "Exec PHP" http://wordpress.org/extend/plugins/exec-php/
I would recommend to the coders to mention this in the section that talks about putting PHP into the page/post, or it just doesn't work.
You can see this has come up a couple of times already in
forum post
forum post