Can't find packages combinat and cluster

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Thanks. That tells me were to start looking. I downloaded 8.1 because the v8.3 dmg is listed as requiring OS X 10.13.6, but I have 10.12.6. The 8.3 installation guide has a long list of packages, but doesn't mention combinat or cluster/quiver. The 8.3 Tutorial doesn't mention combinat either. I'm fairly stuck, anyway; first time I tried to run the program it wanted to change from notebook to "Jupyter", but the tutorial doesn't explain what that is. . . .Learning curve ahead.

I think I can see the source of your confusion. Sage is really two things: Primarily it is a Python library and command-line interface based on IPython (also all written in Python as part of the sage package). If you are not deeply familiar with Python that might not make any sense. But really it's just a Python package, and Sage's language is Python (with a few extra bits of syntax that are used from time to time). Sage is also a big bundle of other software that comprise its dependencies, and the many other software packages Sage provides a common interface to. Those are the "packages" listed in the installation docs. Support for quiver algebras is just part of the core sage package.

Meanwhile sage.combinat just refers to a sub-package of the sage Python package. It's where much of the combinatorics code lives, though not all. But that's mostly an implementation detail not of concern for a beginner. The reason you were having trouble is that the sage-combinat package used to be a separate package from the main Sage package, but not anymore. Your best bet is to always search the reference manual for topics you are interested in (as in the link I gave above). If there exists a longer tutorial for the topic there will typically also be a link to that tutorial, but for many narrow areas of interest there won't be a specific tutorial (someone would have to write it!). But it is good to go through some of the more basic tutorials as well.