In Autocad, under Tools/Load Application,
Find your NHT directory and load in the following files:
Then go to the individual module sub-folder and load in the two lsp files you see. In your case go to the PI directory and load in:
That should get the piping section of NHT loaded. You will have to do this every time you start autocad. I have not tried putting these files in startup suite as I don't need them all the time. I am not sure if all these files need to be loaded, but this is the combination I found that works and don't want to mess with it.
There should be a toolbar for NHT that, on my screen anyway, has question marks for the buttons because there are no bitmaps assigned to them and I didn't bother fixing it because I use NHT intermittently.
Hope this helps.