I'm having the same issue as @Netzialist.
I figured it out on my end, and it might have been the same for others. At the end of the readme, the code you give to add to the functions.php is this:
The easiest way to add a placeholder is to put this in your functions.php
$rotators = array();
$rotators['homepage'] = array( 'size' => 'homepage-rotator' );
Notice that FlexSlider is camelcase, while it needs to be written in all lowercase. Changing 'FlexSlider to "flexslider" fixed it for me.
Thanks for making the plugin!