When adding a plugin to WordPress.org plugin repository you need to host the plugin somewhere (anywhere) and that is the “Plugin URL” so the good guys at WordPress could download it, test it and approve it to be listed.
so just upload it to your host and enter the url to the zip file.
Once it gets approved you will get access to http://plugins.svn.wordpress.org/your_plugin_name/ using your WordPress.org user name and password.
Here’s some handy links to help you
get started.Using Subversion with the WordPress
Plugins Directory
http://wordpress.org/extend/plugins/about/svn/FAQ about the WordPress Plugins
Directory
http://wordpress.org/extend/plugins/about/faq/WordPress Plugins Directory readme.txt
standard
http://wordpress.org/extend/plugins/about/readme.txtreadme.txt validator:
http://wordpress.org/extend/plugins/about/validator/
this is what you get in the “WordPress.org Plugins Request Approved” email notification.
Hope This helps.