By "all files", I mean I copied from the plug-in to the checkout_one and nopt from the store:
jscript_calendar_head.php
jscript_b-calendar.js
jscript_a-mootools.js
This was suggested
here because the calendar was not displaying.
The plug in file I downloaded do not include any header_php.php file. The plug in works great with OPC off. It just won t register the date into the order when OPC is on.