Are you on Linux? The "Play DRM content" checkbox is shown by default on Windows and Mac OS X.
The default for OSX is hidden - http://imgur.com/3AdT6Vs (46.0.1)
On Windows, it is shown indeed (45.1.1 ESR).
Thanks for the correction. I forgot that EME support (using Google's Widevine CDM) won't ship on OS X until Firefox 47, which is still in the Firefox Beta channel for another few weeks.
In order to enable it, you must go to about:config and enable browser.eme.ui.enabled. While you are there, you can just disable media.eme.enabled and media.eme.apiVisible.