Hi,
You will want to create a directory called binomexp attexmf/tex/latex  so to keep the binomexp.sty file, which is what Latex reads when it uses the \usepackage{binomexp} command in the premable of your work.
Put the binomexp.sty file inside of the /texmf/tex/latex/binomexp directory.
Now, if you are using Miktex go to start, all programs, miktex, miktex options.  Now click on the roots tab. Now select c:\texmf using the mouse.  Now click on Refresh FNDB.  You've installed it!! 

You might also want to create another directory binomexp at texmf/doc so to keep the documentation at.

LICENSE
Do what ever you want with this program. Eat it, spend it, sell it, dwell in it.  Use it for hamster bedding.
Subject to the Latex Project Public License.
