To install, add snipurl/hpcalc to Installer.app sources, and find the calculators listed under Utilities. http://snipurl.com/hpcalc is what the source should look like!
However, even though I am of Polish descent and an engineer I never got the hang of RPN. I seem to remember HP made at least one of these calcs with algebraic entry (i.e. an "=" sign). Any chance making one of those?