if you cant able to install by Plugin Manager . than you can do it manually , go to the https://sourceforge.net/projects/jsminnpp/
and download latest zip of plugin and extract it and you will find a JSMinNPP.dll file , and now go to installed folder of Notepad++ , in my case ( C:Program Files (x86)Notepad++) and go to plugins folder and copy JSMinNPP.dll file -
kiran