I tend to use Homebrew on Mac. It will install and configure all the stuff for you.
Link
Then you should be able to install it with brew install mcrypt php53-mcrypt
and it’ll Just Work ™.
You can replace the 53
with whatever version of PHP you’re using, such as php56-mcrypt
or php70-mcrypt
. If you’re not sure, use brew search php
.
Do also remember that if you are using the built in Mac PHP it’s installed into /usr/bin
you can see which php you are using with which php
at the terminal and it’ll return the path.