Before running hnc.gp you need to have pari-gp and LiE on your system.
Download hnc-x.x.z.tar.gz, unpack and go in the directory HNC. Start
pari with the command gp. Pari promt will appear. Check that in your pari 
session the file .gprc was sourced. Now try the command
> hnc(GROUP, REPRESENTATION)
GROUP is semisimple group given by its factors and the REPRESENTATION is given by a highest weight vector, like
> hnc(A3A2,"X1,0,0,1,0")
The output is a list of stata of the null cone in the representation.

Study the source code hnc.gp, which contains explanations.