Simply copy the "DDMathParser" subfolder into your project, #import "DDMathParser.h", and you're good to go. A demo target is included as part of the project. It shows how to evaluate a user-entered string, with support for variables.
Alternatively, you can add DDMathParser as a submodule to your git repository, which will make it very easy to stay on top of the latest changes. This question on StackOverflow.com has an excellent write-up on how to do that.
Supported OS versions
DDMathParser has been tested using Xcode 5, OS X 10.9, and iOS 7. No effort has been made to maintain backwards compatibility with previous releases of the OS or tools. If it works, yay.