I am trying to follow this tutorial to compile the example
addtwo program. But I am stuck on how to include the files
to the project. Xcode seems to have changed enough since the tutorial was written (2008) that it's not clear how to add these files to the project. (I am very familiar with how to write programs in C, but I'm much less familiar with Xcode)
More generally, is there an updated tutorial that explains how to write a C program with MathLink on Mac OS X 10.10+ with Xcode 7+ that will be called from the Mathematica kernel?