The stand alone installer installs DjVu Plug-in only to the user who runs the installer.  If you installed the Plug-in under Administrator,  and if you are running it under another user, you can't open a DjVu file.

This behavior is by design.  If you want to install the plug-in to all users on your computer,  please run following commandline using the commandline prompt.

msiexec /i djvuctrl-6.1.4-en-rXXXXX.msi ALLUSERS=2