Download the package at from CTAN (direct download link), unpack it, extract its contents (wordcount.tex and wordcount.sh) and copy it in the same folder as your document. Let's assume this document is name test.tex, and that it is in the folder /home/work/.
Launch a console, go to /home/work/ using the command cd, i.e., do
cd /home/work/
Then give to your script wordcount.sh the right to be executed:
chmod +x wordcount.sh
You have to do all of the above only once. Once all of this is set up, a simple
./wordcount.sh test.tex
should gives you what you want. Make sure that you can compile your document test.tex without errors, otherwise wordcount.sh might not work.
If you are confused by Mac's terminal or uneasy with it, you could ask at https://apple.stackexchange.com/ or try apple support.
You might also want to compare wordcount's output with other software's count. For this, have a look at Is there any way to do a correct word count of a LaTeX document?