A number of you have noticed that the latest version of ImageMagick no longer contains the mogify command.
An update to LR/Mogrify which uses the latest versions of ImageMagick will be forthcoming, however in the meantime you can continue to use the plugin by installing version 6 of ImageMagick, which is still maintained here: http://legacy.imagemagick.org/script/binary-releases.php#windows.