After downloading the winscp514automation2.zip make sure that Windows has not marked it as untrusted. To do this, right click on the zip file and select properties. Click on the Unblock next to the Security section at the bottom of the properties box if it is there. Now extract the files (WINSCP.dll and readme.txt) and place them in the original folder WINSCP was installed in. If the files are untrusted and you try to register them they will fail. Follow the readme.txt file for how to register the Winscp.dll file.
Using what privileges are you registering the assembly that you need to do the Unblock procedure?