In v7.7, I had to go to Plugins menu and select "Open Plugins Folder..." (which goes to C:\Program Files\Notepad++\plugins).
I had to create a folder for the plugin and extract the .dll into the folder. For example, create a folder called "JSMinNPP" and place the "JSMinNPP.dll" in that folder. It doesn't work if you put the dll into the plugins folder.
Finally go to Settings --> Import --> Import plugin(s) and import that dll and restart Notepad++.