As Claudia says, and as happens to me, Plugin Manager cannor download it from sourceforge.
The PM says its at: “/projects/jsminnpp/files/Uni/JDToolNPP.16.10.uni.zi” (and it cuts it there).
When I go to: https://sourceforge.net/projects/jsminnpp/files/Uni/ I see several files, and “JSToolNPP.1.16.10.uni.zip” downloads fine. And it contains “JSMinNPP.dll”.
I copied the dll into NPP’s plugin directory and re-running NPP shows it in the Plugins menu and it works.