Install SRA tool-kit on MAC OS

Download file to /Users/your_user_name/Downloads/ folder:
http://ftp-trace.ncbi.nlm.nih.gov/sra/sdk/current/sratoolkit.current-mac64.tar.gz

Extract compressed folder:

tar -vxzf  /Users/your_user_name/Downloads/sratoolkit.current-mac64.tar.gz

Add exeacuteable to your env path:

export PATH=$PATH:/Users/your_user_name/Downloads/sratoolkit.2.6.3-mac64/bin/

Check:
which fastq-dump
This should show: 

/Users/your_user_name/Downloads/sratoolkit.2.6.3-mac64/bin//fastq-dump

For SRA tool-kit installation on other operating systems, check:
https://github.com/ncbi/sra-tools/wiki/HowTo:-Binary-Installation