If you are using the demo data with eZpublish 3 and will be creating new content based on new classes you may need to change the permissions for the user section of your site. On a fresh install with the demo data the user is only allowed to view the specific classes that come with the demo - extra classes you may create will not be viewable unless you relax the permissions for the anonymous user.

Clicking on Roles within Admin will show the current list of roles. Editing the Anonymous role will list the current restrictions for the typical user. You will notice for the content that only the shown classes are visible to the user. To alter this create a new policy for the content that allows either all classes to be readable (content/read/*), or select those that are relevant to you. After creating a new policy you may need to delete the previous policy, so as to avoid confusion. Once done Apply your changes.