Set the dropdown that says "Sequential" to "Full Random" and adjust the NotesAbove and NotesBelow... It'll do what you want, but quality may vary. The script is made for varying repeated notes in single-sample instruments, not for fully random sound effects.
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You could also set up a sampler to use velocity switching so louder footsteps are triggered when you press harder (or grittier, or more heel-to-toe, or whatever you want the main variable to be, if any, or just random). Given MIDI's resolution, you could therefore dial in up to 127 different footsteps based on note-down velocity with a single key, and be able to change them by just changing the velocity setting of each note (a snap with most MIDI-aware DAWs).
if you don't find a sampler to your liking, you can go with a video game development tool like WWise. These tools are free to download and use, fit those kinds of needs well, and are very fast and flexible. The companies make money when you license them for implementation into a game.