I have solved the problem, or at least the problem is solved. Maybe I had nothing to do with solving it. But call off the developers. I tried again today, and today I was able to register. I don't think I did anything different, so maybe my computer was on vacation. On the other hand, maybe I did do something different.
I will say that it is not intuitive which box one should try to paste into, the large upper one or the small lower one. Once a key is pasted, it is clear where to enter data. But for the first entry it could be labeled "enter code here."
I received the Free License Key via email. I copied this to Windows Wordpad (I am using Windows 7). I try to paste file copied from Wordpad into the License Key Storage window of my Viewer ( Version 126.96.36.199) , but the window does not allow the cursor to enter, and so doesn't allow anything to be pasted. I deleted the config_4.xml file, exited Viewer and tried again. Same result. Is there a workaround, for example can I paste the key, using regedit, in whatever is the correct Registry file?