[Snip]

- Click the "Administration" link in the XWiki toolbar

You can access the Administration toolbar by moving your mose over the small icon at the top left of the page (the tools on a transparent background, you maight not see them if you're not using Firefox), on hover the menu will unfold and provide you with the "Administration" link Jérome is talking about (which is different from the "Global Admin" link on a red background).

- Click the "Space rights" tab in the administration page
- Select your organization top-level space in the drop-down list (the one
without any suffix: Space_MyOrganization, not Space_MyOrganization_Wiki or
Space_MyOrganization_Blog)
- Once its loaded, click twice the checkbox for unregistered users under
"view" to have it green-checked (it should appear with a red circle at
first).
- ... and your done! The _Wiki, _Blog, etc. spaces will inherit their
rights from that top-level one, so you shouldn't have anything to do for
them.

If you want to have registered users able to comment, you can check the
"comment" box for XWikiAllGroup as well.

Hope you'll manage to achieve your objectives... We'll be glad to have a reference from you added on http://www.xwiki.org/xwiki/bin/view/References/ once you're done ;-)

Guillaume