Aubrey Portwood on "Unable to use register_activation_hook"

I have been trying to use https://gist.github.com/aubreypwd/2fc7fd885306e7f48ec0 to flush rewrites on plugin activation to no avail.

This is for http://wordpress.org/plugins/itempress/, which is a plugin that simply adds an ambiguous custom post type. But, on activation the flush isn't happening, so I am trying to code in the above script so that the new URL's work.

Any help would be great!