Here's more info about the installation on my 97 Pathfinder:
Airlift Air Spring Installation on 97 Pathfinder
FYI- the basic kit (without the compressor) is easier to install than the compressor/gauge. The hardest part about installing the compressor, gauge, etc. is finding a location to put everything.
I do not recommend placing the compressor on the exterior (underneath) the vehicle, because it is not well-sealed and would be subject to exposure to water, dirt, mud, etc. causing early failure.