I seem to be getting the following when I execute npm install bower -g
/usr/local/share/npm/bin/bower -> /usr/local/share/npm/lib/node_module
For users that are encountering issues with the installation in mac as shown in the official page, it seems that El Capitan is giving permission issues to install the package in that way:
npm install bower -g
The solution I've found to avoid the permission errors is using sudo (superuser do) to provide access for node to download the package like this:
sudo npm install bower -g
Hopefully this may help users having the same problem. :)