Very logical that it jumps to the first slide. Seeing the labeling of the variables, I think you refer to a post where I answered or that was based on one of my blog posts.
The bug in your script is this one: rdcmndGotoFrame = rdInfoCurrentSlide - 1
Have a look at my blog post about system variables: http://blog.lilybiri.com/system-variables-in-captivate-6
You'll read that rdInfoCurrentSlide is an index starting with 0, so in your 'calculation' the result will be -1. But CP doesn't recognize negative, so will think it is 1. What you are telling CP is to go to the first frame of the movie, which is the first frame of the first slide. So it will go to the first slide.
If you just want to rewind one frame, you'll have to use:
Expression rdcmndGotoFrame = rdInfoCurrentFrame - 1
Apparently you are on version 5.5 or 5 because the rd-variables have disappeared from 6 on. Can you confirm?