The syntax would be [1d6hk-1] you can test it by doing [1d6hk-1] [1hm] for a fixed multiplier.
That will result in a possible value of 0 but I am reliably informed thats the correct calculation

See this thread in the Hero Forums
Syntax for a typical killing attack would be something like [1d6hk] [1d6hm-1]
A multiplier of [1d6hm-1] will never result in a value less than 1, as per the rules.
I've attached my latest version. Its pretty much the same except that the regular expressions for the parsing is a bit neater. As above you need to unzip the file then rename the "dicelib-1.0.b20.jar" file to match the DiceLib.jar in your distribution, probably "dicelib-1.0.b18.jar"