Question: how to install wgrib2 on mac?
Answer: Get Command Line Tools for Xcode (search for it on Duckduckgo or your engine of choice) and also make sure gcc is installed. ... 
Download wgrib2 here (note download links are pretty far down the page).
Untar the tarball somewhere, and roll up your sleeves. ... 
In the makefile, uncomment the lines.