In my Macbook Pro in .txt file if I type رَبِّ. It shows incorrectly. The 'zer'/ 'dhamma' show upper of ب but it should shows lower of ب.
can anyone help me to show it correctly.
Technically, I don't think your Mac is incorrect here. At least in formal/Modern Standard Arabic (which is all I've studied, and I'm not an expert), it's a convention to put shadda and kasra together above the consonant.
Nevertheless, try switching fonts. Different fonts follow different conventions for the placement of 'extra-alphabetical' symbols. For example, on my Mac, the Mishafi Gold font puts the kasra below the baa as you want. Or, if you're using a regional variety of Arabic that follows different conventions, try selecting that specific variety in System Preferences > Language & Region, if it's listed. That might make a difference.