This happens when a changeset has been pushed to Kiln but no case number was put in the commit notes. So I would go to the changeset and manually add it.
As I mouse over "Cases (0)", it changes to "Add a case". Clicking this doesn't seem to do anything. If I use Firefox it shows a small box underneath which allow details to be entered. If a changeset already has a case linked to it, then this shows a different screen layout which does work in Chrome.
Edit: As pointed out by M. Esh in the comments, this is not an issue with Chrome 7.0.517.41 with any of the extensions enabled as mentioned in the accepted answer below.