Drupal install insists on using default sites folder

Seeking suggestions about a multihoming setup. Drupal is not finding my site's folder to install itself. I keep thinking I've figured this out once and for all, but no...

Setup: Drupal 6.20 on OSX 10.6/Apache 2, multihoming (multiple hostnames and IP addresses) and multiple Drupal codebases. I'm admin.

Site folders are like so:
/DocRoot/mysite1-drupal-6.20/sites/mysite1.mybox.com (mysite1.mybox.com has its own IP address and its own Drupal codebase)
/DocRoot/mysite2-drupal-6.20/sites/mysite2.mybox.com (ditto), etc.

(I just like doing it this way, and it's worked well for me in the past.)

When trying to install, for instance, mysite2.mybox.com, I get:
"The Drupal installer requires that you create a settings file as part of the installation process.
Copy the ./sites/default/default.settings.php file to ./sites/default/settings.php."

Database is set up, permissions are ok. Apache is routing mysite1.com to correct folder. (If I set up a flat mysite1 site, it works)
But seems like Drupal will only look at sites/default.

httpd.conf points mysite1.com to /DocRoot/mysite1 (which is a symlink pointing to mysite1-drupal-6.20).
mysite1-drupal-6.20 sites folder:
/default
/mysite1.com (copied from default/)
--- settings.php (copied from default.settings.php)

This feels like a FAQ but I have not found an answer. I keep finding references to 'multisite' installs sharing a codebase, or localhost installs.

Thanks in advance for any suggestions.