Hi @HowardC, welcome to the forum!
The How to Build a Brain exercises can be found as IPython notebooks here: https://github.com/nengo/nengo_examples/tree/master/htbab_tutorials
Note that those exercises require Nengo GUI, despite being run in the notebook. If you want to run the exercises directly in the notebook, they are available in a slightly modified form here: https://github.com/nengo/nengo_gui/tree/master/nengo_gui/examples/hbb_tutorials
Those exercises will be available directly in the GUI when we do the next release of the GUI, which we're working on! No clear ETA at the moment though, so the links above will be your best bet right now.