Thanks much for the suggestions so far. The hosts file had been updated, else I would never have gotten mainsite.dev to feed me the WP's mainsite content. But I had not done any mapping of the subsites as suggested. So took care of the subsites in hosts and vhosts.conf. Unfortunately, the situation remains unchanged. WP only serves up mainsite.dev, with just the directory listings for the subsites, which are physically empty.
One thing I did notice in keesiemijer's settings' suggestions above, is that it has mainsite as a subdirectory in Sites. I've got everything at the Site's root, figuring that might be less prone to error. Will try moving everything one level down and see if that triggers any changes?
As noted earlier, I went this rout as the MAMP solutions above were not working for me (have used Single User WP on MAMP successfully for years). Figured I would give the native Snow Leopard package a try--again noting that it's a bit harder to get the Apache/Php/Mysql thing set up than with MAMP. But once up, figured I would be no worse off. And indeed, I'm visibly further along, as with MAMP I could not get the mapping to even serve me WP's on main site for installation. Also, if this could work, be another set of options for folks. But close is not close enough, so maybe I will need to go back and try again with my MAMP configurations.