{{checkbox|0}} Its download page is http://nsis.sourceforge.net/Download and you should download the newest version (currently 3.0rc10) and not the earlier version (currently 2.51).

But when you install it, don't accept its suggested installation folder if that folder path has a space in it, since (in nsis-3.0rc1 0 or nsis-2.51) there is a bug and it cannot cope with such a path (when running in CLI mode, to my amazement). Instead, modify the suggested folder name to have no space in it -- so I changed C:\Program Files\NSIS to C:\NSIS instead. (Since I didn't know what was needed, I left the suggested type of installation at "Full". You can uninstall NSIS when you're done.)