Add the .sty file to either the folder where the latex document is, or to the appropriate part of your texmf
directory, which is found at:
Windows: C:\Users\<user name>\texmf\tex\latex\local\
Mac: /Users/<user name>/Library/texmf/tex/latex/local/
Linux: ~/texmf/tex/latex/local/
Then, just need
\usepackage{mathmacros}
before \begin{document}. I think you might need the package amsmath, I didn't really go through and play around
with the dependencies. Feel free to propose additions to the file; these are just the few I've used relatively recently.