Make sure you have gitk installed:

$ which gitk
/usr/local/bin/gitk

If nothing shows up, then gitk is not installed on your system, or at least is not available on your $PATH.

Change the directory to the Git-Version-Control-Cookbook-Second-Edition repository from the objects and DAG examples. Make sure the master branch is checked out and pointing to 13dcad:

$ git checkout master && git reset --hard 13dcad