You need to open up the File window in winbox, and drag the file to the blank spot in the window - not the menu itself – so that .npk package appears at the root directory inside Files dialog box. Then do a System > Reboot.
You check Logs to see if has any messages after reboot after copy’ing the UM package.
Also may want to check System > Resources and check that you have enough free disk space for the package (although there be a log message about disk space in logs). Most routers are fine, but if it’s a 16MB flash that’s problematic for additional packages.