Many thanks to Stephan Eggermont, Tobias Page, Esteban Maringolo and John McKeon for their responses on the SmallWiki and Seaside lists.
It's clear to me now that my problem was trying to load Seaside, Magritte and Pier Configurations without fully appreciating the details of Metacello and of Configuration loading options.

The method, #asComponent, wasn't missing; apparently I had loaded a configuration that didn't include it.

For now, I will use the forum image suggested by Stephan for my current study of Magritte, and then review further the details of Configurations and loading options.

Thanks again to all of you for your patience and guidance.

Bruce