Make sure to meet the Requirements. Once you have the required software and you can compile the libnds samples from devkitARM, it's easy going.
You solely have to extract the N3D .zip archive, which you probably just downloaded, to your devkitPro installation root folder. You should then find a "libn3d" directory there.
The libn3d directory contains all relevant N3D files, such as C++ header files, library files, utilities and this documentation of course.
1.5.3