Done.
I installed all the files in domain.com/ root. (Somewhere I read to install the directory and rename it wpmu, but then all subdomain blogs are .com/mpmu and I didn't want that.)
Then contact support to setup wildcard domain. (Going through steps without understanding isn't fun, but it worked.)
The DreamHost Wiki is very clear about that. [1]
Then, according to that wiki page, I CHMOD the root directory and /wp-content to 777.
No joy ... 500 error.
Operating blind I created subdir.domain.com and pointed its directory to domain.com ... and that worked!
--bentrem
p.s. I can't find how to join as a new-comer, not logged in, which is the point, but that's for another thread. Moo docs are really skimpy.