How can I install a plugin
From NSIS Wiki
Plugin general information
Plugin is a way of extending application's functionality by adding new functions. For NSIS plugins consist of two mandatory files (with extensions ".dll" and ".nsh") and some informational files ("readme.txt", "license.txt", help files with ".chm" extension). Files with ".dll" extension hold bodies of plugin functions in form of Dynamic Link Library (DLL) and ".nsh"-files (header files) hold some data in plain text for using that functions with NSIS.
"NSIS searches for plug-ins in the Plugins folder under your NSIS directory and lists all of their available functions. You can use !addplugindir to tell NSIS to search in other directories too." -- NSIS manual, chapter 2 "Tutorial: The Basics".
NSIS plugin installation
After downloading and unpacking a plugin one needs to accept the license agreement (residing in "license.txt" file). Then the ".dll" files must be put into "NSIS/Plugins" subfolder of NSIS installation and the ".nsh" files -- into "NSIS/Include" subfolder.
Dialogs plugin installation example
"Dialogs" plugin is distributed in a form of ZIP-archive named "Dialogs.zip". The archive contains 4 mandatory files: "dialogs.dll", "defines.nsh", "ZipDLL.dll" and "zipdll.nsh". The last two forms ZipDLL plug-in which is used by the "Dialogs" plugin. All ".dll" files must be extracted into "C:\Program Files\NSIS\Plugins" folder and ".nsh" files must be extracted into "C:\Program Files\NSIS\Include" folder, supposing that NSIS installation name was the default "C:\Program Files\NSIS".