PluginData is the folder that plugins use for their data files. The correct folder for the plugins themselves is "Plugins" not "PluginData". So, in your case, it would be:
C:\Users\username\Documents\The Lord of the Rings Online\Plugins
(you shouldn't ever post your actual username, even in document paths)
It is usually easiest to install and update plugins with Lunarwtr's Plugin Compendium:
http://www.lotrointerface.com/downlo...nfo.php?id=663
or for Mac/Linux clients:
http://www.lotrointerface.com/downlo...mMacLinux.html