You shoule have five files: Makefile, ast.ml, lex.mll, parse.mly, and interp.ml. Assuming you have make and OCaml installed, typing "make" should create an executable called interp.
If you spend more than 2 minutes on this, let us know and we can just email you the five files.
By the way, the name tar is a great anachronism, but not nearly as good as the "cc" field in email headers.