The XML file is correct for the compatibility with Joomla 1.5.
You mentioned that you have a white screen and no message displayed.
Based on this info, this means that you have something specific with your environment and if you are sure that you can upload a large file, I can just suggest that try from a directory as I explained to you.
Otherwise, I don't know what is specific in your environment that would forbid the installation.
If you want to test it on a hosting company that has no issue, you can try with the shared web hosting at hostgator.com
If you wan that I have a look on your environment, this is also possible with our billable support that you can order at
www.jms2win.com/en/joomla-multisite?page...35&category_id=1