Once you have downloaded and unzipped the package. There is already a precompiled executable which can be found in:

STAR-master/bin/MacOSX_x86_64

So, either explicitly state the path to the executable or export the path (if you are using bash in MacOSX):

export PATH=$PATH:/path/to/STAR-master/bin/MacOSX_x86_64

That should work.

An easy way to install STAR would be to use Homebrew, in case you don't already have it. It not only have Unix utilities you can download but there a plethora of science tools also available. Once you have Homebrew installed, installing STAR can be as easy as

brew install homebrew/science/rna-star

I hope that proves to lower the barrier to at least playing around with STAR.

Otherwise, I feel that their instructions were pretty clear.

# Get latest STAR source from releases
wget https://github.com/alexdobin/STAR/archive/STAR_2.4.2a.tar.gz
tar -xzf STAR_2.4.2a.tar.gz
cd STAR_2.4.2a

# Build STAR
cd source
make STARforMacStatic