How to get WP Smush Pro to work?

Hi,

I upgraded to WPMD (or something like that) so I could use WP Smush Pro but when I attempted to use it I got the following message:

Request Entity Too Large (HTTP Error 413). Try sending lesser number of images, you can use define( 'WP_SMPRO_REQUEST_LIMIT', 500 ) to send less number of images

I can find no other way of using the software. It indicates going straight to your media library and using it in there but there is no option in the library to use it.

Is it possible to use it at all?

Thanks