I've uploaded notes for lecture 4 along with a polished up version of the GMPL model for doing the Critical Path Method (CPM) calculations. Hopefully the notation is more consistent throughout the notes, and translates directly to the GMPL file.
We're becoming somewhat more sophisticated users of GMPL, so you'll note that the model and data files are now separate. For the assignment you should be able to create a new data file describing the problem and obtain needed results with few (if any) changes to the model.
Project management is an industry of its own, and you'll find a wide variety of software tools to support project management. Software ranges from simple charting all the way to tools for planning, scheduling, and controlling mission-critical corporate-scale endeavors. If you're interested in this topic, you might try to try your hand at GanntProject, which is a cross-platform, GPL licensed tool for small scale desktop project management. One option is to run it right from the web, so you can try it without having to fiddle with software installations.