NextGEN Download Gallery v1.5.1 has just been released. It fixes a compatibility problem with NextGEN Gallery 2.1.7 and later.
NextGEN Gallery 2.1.7 came out today, and one of its changes meant that the Download Gallery’s “download all images” button disappeared. The “download all images” button can be enabled in the Gallery settings. It relies on the gallery ID, and NextGEN Gallery has changed how that is obtained.
You can update to version 1.5.1 from your WordPress plugin admin page. Here’s the full changelog for NextGEN Download Gallery.