Installing latexdiff on a Mac (without having MacTeX installed e.g. if you use an online LaTeX editor):
Install Homebrew, the missing package manager for Mac ->
Install perl -> brew install perl (N.B. this step might not be necessary)
Install latexdiff -> brew install latexdiff
Keep latexdiff up-to-date -> brew upgrade latexdiff
You may need a .sty file, which can be extracted from a .dtx by doing the following:
Go to the folder where the .dtx file is located
Run tex file_name.dtx
This should generate some other files on the current directory, one of which is a .sty file.
You can then move the file_name.sty file directly into your project folder, where a file will use it, or ...