Plugins for XnView come in form of DLL files. These DLL files are usually found in ZIP archives posted on the Support page: http://newsgroup.xnview.com/viewtopic.php?t=11715
After you have downloaded a Plugin or Add-on, extract it and place the DLL file to its corresponding directory: Plugins or Addons. Reload XnView and you should have access and support for that plug-in.
There is also the possibility of downloading an automated package that installs all the available plugins at once in their correct directories. You can obtain it using the link below.
Plugins download: http://www.xnview.com/en/xnview/#addons
Add-ons download: Same as above (right side of the page).
