VLC says "No suitable decoder module. VLC does not support the audio or video format "IMM5". Unfortunately there is no way for you to fix this".
Thanks for checking - at least I can quit obsessing about how I should be able to get it working.
In case this thread may help others, this DVR is part of an inexpensive 4-camera system made by Samsung and sold by Costco. The model number is SDE-120. In addition to controls on a directly connected monitor, it has a nice web-enabled control interface that only works completely with an Active-X control that runs under Internet Explorer. The live web stream can be viewed on Mac OSx with Firefox or Safari, or Safari on iOS, but without the ActiveX control it shows only one camera at a time and users cannot view or control playback of recorded images.
To be fair, the specifications for this device do say that it will work only with IE on Windows, and the fact that it does provide limited viewing on other browsers is not claimed. I also cannot find any specification of requirements for playing the saved "backup" file.