Installing custom content on NWN2 is made somewhat complicated by the fact that every platform and OS has its own way of storing application programs and user content. I'm listing here the default locations of these so that when downloading custom content it will be more clear where it should be installed.
NWN2 always installs two sets of very similar folders. One set is for official content and one for user specific content.
One set of the folders is for official game content. This set is always associated with the Application, so I'll refer to them as the "Application folders". Modifications should almost never be made to the folders and files in the Application folders. This was more important when the game was still being actively patched because custom content in the Applications folders could break the patching system. There are a few exceptions to this including the OC Makeover SoZ Edition and MotB Makeover SoZ Edition.
A second set of folders is installed to allow user modifications and these are generally installed in some kind of user specific Document location, so I'll refer to them as the "User folders". Custom content is almost always installed in the User folders. When something should be installed in the "campaigns", "modules", "override" or "hak" folder these are referring to subfolders within the User folder.
Windows XP standard installation from game discs:
App folder: C:\Program Files\Atari\Neverwinter Night 2\
User folder: C:\Document and Settings\[user]\My Documents\Neverwinter Nights 2\
Windows 7 standard installation from game discs:
App folder: C:\Program Files (x86)\Atari\Neverwinter Nights 2\
User folder: C:\Users\[user]\Documents\Neverwinter Nights 2\
Windows XP/7 installation from Steam:
App folder: C:\Program Files\Steam\steamapps\common\neverwinter nights 2\
Windows XP User folder: C:\Document and Settings\[user]\My Documents\Neverwinter Nights 2\
Windows 7 User folder: C:\Users\[user]\Documents\Neverwinter Nights 2\
Mac OS standard installation from game discs:
App folder: Applications/Neverwinter Nights 2/Data/
User folder: Users/[user]/Documents/Aspyr/Neverwinter Nights 2/
Mac OS standard installation from Mac App Store:
App folder: Applications/Neverwinter Nights 2.app/Contents/Data/
(to open this right-click on the Neverwinter Nights 2.app icon and select Show Package Content from the context menu)
User folder: Users/[user]/Library/Application Support/Neverwinter Nights 2/
(to open this start the Terminal utility and issue the command "open Library" (without the quotes))