Hey Will!
The script that you download from the resources section is called nhpexpirator.txt.
It say rename it to nphexpirator.cgi - as it is a nph file on my server at least I had to call it nph-expire.cgi. The expire bit I choose and it worked when I ran it.
Does it replace nph-build - do not think so, have a look at the code contained in the file..
I got an error message when I first tried to run it. it is mentioned above and PaulW suggested I comment out that line in the script. I did and it worked.
Hope that helps.