I know this is a little late(several years) joining the conversation but I was also trying to run a stepper motor with some LMD18200's that I had left over from an old project when i found this post. The datasheet clearly says the chip can be used for a stepper and THIS IS TRUE! However, I think the chip was mainly intended for use with a brushed dc motor (dir, pwm, etc).
To run a single stepper motor you need two LMD18200 chips and a way to control the outputs. I used a mega2560. The hard part is figuring out what digital inputs to send to get the right motor output states. The mapping is counter intuitive IMHO but the truth table describes it correctly.
Mine is working great and 'half-stepping' up to about 150 steps/sec without missing steps. Sorry for the poor video quality. Please let me know if you have any questions.
Video on YouTube: lmd18200 stepper - YouTube