On OSX should NPM install global packages to:
/usr/local/lib/node_modules
or
/usr/local/share/npm
The global packages get installed to:
/usr/local/lib/node_modules
and man pages get installed at (if supplied):
/usr/local/share/npm
see this for more information.
The directory will differ depending on whether or not you're using nvm or some other node version manager, and possibly also based on versions of version manager and versions of OSX. Due to the variability here, it may be better to run npm root -g which will specify the directory npm is using to store your global modules.