there really isnt an easy solution for the mac.
the PHP server that is built into the MAC OS is not compiled with GD library support.
you would need to recompile the php server and include the string to compile it with GD support.
the MAC OS kernel is based on Linux, so the compiling php for mac is very similar to compiling it for linux. Here is a tutorial for compiling php fior linux:
sorry, i could not find one for mac.
the Simplest solution is to not use the MAC OS PHP testing server, and use MAMP instead, the mamp PHP server has been compiled with GD library support.
in reality, the user registration solution pack is intended to be uploaded to your host webserver so that site visitors can register to your site.