I am new here and was having the same problem. Your question and the replies nudged me to do some experimenting. I hope this helps you, too.
First, make sure you are using Internet Explorer. Other browsers do not play well with ActiveX.
Next, you will have to go to your Tools > Internet Options settings and pretty much tell Windows and IE to let ActiveX stuff run (or at least prompt you). After I made the changes, instead of getting the "...terminated, unable to verify..." error message, I got a prompt that sort of said "We don't know the publisher. should we run it anyway?" and I told it to run it. Voila!! I can get to the DXClient netviewer without running the software my DVR came with. Anyone with IE that is willing to change their ActiveX settings should be able to see the same display. After the software is installed, you should be able to go back to your previous ActiveX settings, since all you wanted was to be able to install the .cab file.
Again, you are limited to IE, but at least it works.
Diggs
UPDATE: There is a plug-in for Firefox that will let this run. Get IE Tab and "open a new tab in IE Tab".