The popup of a TinyMCE is outside WordPress. You must include the wp-load.php, same handle like outside the WordPress install.
But think about this solution and maybe it is cleaner, that you create your data in a json string and handle this data in the popup. It is helpful to reads this post about the include od wp-load.php.
Here you find a Gist with a example to write a TinyMCE popup dialog in WordPress, without to include the wp-load.php. The author have also write a tutorial, but only in German language.
Also the hint to this WPSE Question about a tutorial. This tutorial create the popup content only with javascript. Maybe more secure and the data for the handle will also communicate via js to the editor.