What has happened there, is for some reason the forums application hasnt been unpacked. What you need to do is the following
Upload a fresh set of files from your client area, in order to ensure that the forums upgrade files are now present
Go to yoursite/admin/upgrade and complete the upgrade
Once done, you will likely find that application is disabled, so go to System>Site Features>Applications and select to enable it again
This should resolve your problem, but of course if you have any further issues, let us know