WPMU install on add-on domain

Hi,
I still have a problem installing WPMU on add-on domain.

eg.
my server main domain = http://www.abc.com
and I decided to install WPMU on new domain = http://www.xyz.com
so I had add-on http://www.xyz.com domain on my abc.com hosting.
on my server view the xyz.com using subfolder name public_html/xyz
but if I login to xyz.com ftp account, it should be a root directory for its domain.

I try install WPMU on the xyz.com domain with $base = "BASE", but failed.
same result when I set them to "/" or "xyz",

Can anyone help me on this issue?
Thanks