The content of this iteratee-diagrams package follows the same hierarchy as dia's usual data directories. The sheets directory contains the file Iteratee.sheet, which creates an option called Iteratee in the Dia sheets user interface. The Iteratee sheet specifies the shape and icon files for the iteratee shapes, which are stored in the shapes directory.
You can install these custom shapes into your user dia directory, usually ~/.dia:
$ cd iteratee-diagrams $ mkdir -p ~/.dia/sheets && cp -r sheets/* ~/.dia/sheets $ mkdir -p ~/.dia/shapes && cp -r shapes/* ~/.dia/shapes
or into your system dia directory (usually /usr/share/dia).
Then, start dia:
A sheet called "Iteratee" will appear in the main dropdown menu among the existing sheets like Flowchart and Network.