Joomla 1.5 is an alpha version.
As such, it is a work in progress that should not be used for a working site.
It has been released for testing purposes only.

The SVN is a collaborative system that lets developers modify the code and all users get the version as is at the time of download.

Alpha was released quite a while ago and 1.5 has been largely modified since. Make sure to verify the latest build version and your own version when you report bugs. They could already have been fixed.

To get the SVN this is what is required:
First register on Joomlacode.
http://joomlacode.org/

Please refer to this excellent tutorial on using eclipse:
http://forum.joomla.org/index.php/topic ... l#msg50662

Get yourself a tool such as tortoise svn from: http://tortoisesvn.tigris.org/

The repository for 1.5 is:
http://joomlacode.org/svn/joomla/development/trunk/

1.5 nightly builds are available here:
http://dev.joomla.org/content/view/17/60/