I just installed the evaluation version of Php Ed. It runs fine on simple scripts, but one of the scripts I need to debug users the multybyte string extension (mb_substr function), and it is not contained in the PHP bundled with Php-Ed. How do I enable it?

In principle, you can tweak and extend the bundled PHP by editing the php.ini (located in your C:\Program Files\Nusphere\PhpED\php5 folder or similar). However, for a special configuration I personally prefer running a 3rd party web service (e.g. IIS or Apache).