The main files and folders you will find in the source code are as follows:
qti-eb.php is the main file used to process QTI. You call one of its functions, to process a QTI file - but before you call it, you must define the functions which handle each of the QTI file's elements. This is normally done using a "functions file" such as the ones contained in the functionsfiles folder - see functionsfiles/TEMPLATE.inc.php for the template for creating these.
So the normal procedure is:
index.php is simply the main page as used in the live demo, performing this exact procedure (once you've chosen a functions file and a QTI source file for it to use).
qtifiles contains various QTI files we've been using to test the system. They are plain old XML files from various sources.