As I've said, you are not using the current version.
Since all our installers automatically gets it online, when they run, the issue is not that you downloaded a wrong or old installer, the issue is: what is wrong with your system, that is preventing the installer to get the latest version ? Which is 2.0.0.19, BTW so, you are 2 versions behind. Most probably, you have the old version of the .DLL (this is shown by the error message), but other files were probably updated, and that is causing the error.
An explanation, is that your antivirus is blocking the file after it gets downloaded, so you are still running the old version, because the antivirus is blocking the file and is preventing to copy it in the FSX folder. In this case, the issue might be that you haven't turned off the antivirus properly when installing. Turning off the antivirus doesn't mean just quitting its user interface, but it requires to turn off the service that does the real time protection.
Another possible issue, is that you have a firewall that is preventing to grant online access to the installer, so it can't download the correct files. However, you should have seen a warning about it, as if there was no internet connection.
Have you tried installing the Stand-Alone Addon Manager ? It shouldn't do any difference if the problem is the antivirus but, at least, the last version of the .DLL is also included as a local copy so, it should work even if a firewall is blocking online access.