It's a Java executable, so all you do is double click it to run it (assuming you have the Java Runtime Environment installed). After that you simply browse and load the NCS file you want to decompile. When you close the file, it will ask if you want to save it as an NSS.
You need a copy of nwscript.nss in the same folder, but I would advise making two copies of the program in two separate folders. Set one up with K1's nwscript.nss and the other with TSL's nwscript.nss.