Install Unison, simple way
The difference is that you are using a pre-built package under Linux but are looking at the compile-it-yourself instructions (which would work both for Linux and for OS X).

There are at least two options to solve that:


Use one of the package managers available for OS X, e.g. Homebrew and install Unison by running

brew install unison


You will need Xcode installed as well for this
Follow the "Download" link on http://www.cis.upenn.edu/~bcpierce/unison and download pre-compiled versions from here.