If you didn't upload the content during the upgrade process...
 

It seems that* unless I'm mistaken* if you didn't upload the default sample content to the CMS when you were performing the upgrade from v3.8.x* you're basically out of luck. Is this not the case? Upon trying to upgrade to RC4 again I found that it was not possible to get the CMS sample data integrated into the forum database. There should be an option to import or rebuild the sample data.