It seems that your using PHP is not 5.3 or above. Probably 5.2.x but 5.2’s official support had ended. Please upgrade PHP first.
What version of PHP are you using? The plugin supports ver 5.3 or above, and it seems that your environment can not understand __DIR__, which was introduced at 5.3.
If your PHP is 5.2.x or earlier, please consider to upgrade it. If that happens even with 5.3.x, please let me know.
I added an information on the admin panel to tell why you can not customize maximum embed size from WordPress 3.5, with a reference to workaround. Please upgrade the plugin version 1.0.3.
Prior to WordPress 3.5, you might change the auto-embedded app info box at “Settings” – “Media”. If you set smaller size than your theme body width, it was reflected. It was not only for this plugin but for all embedded media such like YouTube.
Now I noticed that the version 3.5 made it impossible to customize. So you may not be able to customize the width of the embedded element.
