flashes come in different models made to work with different brand names.
you might have bought the canon or sony version or something. you shouldnt really get a third party flash anyway, just stick with the brand of your camera.
d100s are real old. the electronics inside the flash are probably up to date with the newest version of TTL meter that comes in all the newer cameras. d100 is using an older version so thats most likely why it wont work.