Andrew,
I just checked your copy and verified that I get the same error here. However if I go to
Velmex Controls and download COSMOS that copy installs just fine.
This leads me to think that somehow you got a bad download and each download after that is really pulling from the cached bad file.
Give this a try.
1.) Delete all downloaded copies
2.) Run Firefox and go to "Tools--->Clear Recent History---> and select Today for the time range (or clear all if you do not care about recent history)"
3.) Clear the history and then go to
Velmex Controls and download a fresh copy and see if that works?
(Try saving it to a folder that you create, that way you know you have permissions to run from that folder)