The program lies within Developer Tools, more precisely Webmaster Tools.
From the developer:
SimpleText Compiler is a text processing tool that takes ordinary text files as input and creates correctly formatted SimpleText files. It will: Format the text based on HTML-style formatting commands such as bold. Find and replace user-defined macros through the entire text. Remove any from the text.
You may want to check out more software for Mac, such as Genome Compiler, Style or koala, which might be related to SimpleText Compiler.