Everything is running on the VM -- it's running CentOS, and the MySQL instance is also running on the VM. If I log into MySQL via the command line, it understands a host of "localhost", e.g. this works:
mysql -umyuser -p -hlocalhost modx_db
The webroot is defined inside my httpd.conf file -- I'm not using virtualhosts on this yet, I'm just navigating to the VM's IP address (it's running in Bridged mode, so it gets its own IP address).
I did get a more specific error (which I think illustrates exactly what Susan mentioned):
Fatal error: Class 'xPDODriver_' not found in /mnt/hgfs/revo/html/xpdo/xpdo.class.php on line 1823
I could manually write the config file at this point... but I'm worried if the setup doesn't complete normally, I may be missing something.