I assume that you already have Joomla 1.5.0 beta installed on your server.
And for your convenience here are some tips how to play smoothly.
Before you install any extension make sure that the required directory are writable.
To check the permission click on ( Help => System info ).

To support older version of Joomla extensions such as template, components, and modules
enable the “System – Legacy” go to extensions => plugin manager.

Here are the list needed to be writable:

administrator/backups/
administrator/components/
administrator/language/
administrator/language/en-GB/
administrator/modules/
administrator/templates/
components/
images/
images/banners/
images/stories/
language/
language/en-GB/
language/pdf_fonts/
modules/
plugins/
plugins/content/
plugins/editors/
plugins/editors-xtd/
plugins/search/
plugins/system/
plugins/user/
plugins/xmlrpc/
tmp/
templates/
Cache Directory /home/jdsits08/public_html/cache/
Cache Directory /home/jdsits08/public_html/administrator/cache/

I may update this tips from time to time. More tips coming soon….