There should be a PHP folder in the directories you listed above. Inside this directory will be a version number directory. In that directory will be the php.ini.
ie PHP\v5.4\php.ini
Alternatively create a new php file in webmatrix and add the following line into it:
<?php phpinfo(); ?>
Run the page and you will get a list of php environment variables. One of them will be the location of the php.ini file.