Last month I hit this issue on customer site: I was using Aggreagation Design Wizard to do the usage based optimization design, but the "Next" button in the "Specify Object Counts" page was grey even I pressed "Count" button.
What you need to do if you hit the same problem is: Take a look at the dimension list to see any dimension has red line under it, open it to see which attribute has red line under. Input whatever number to update the count of this attribute, then the under red line would disappear.. And the "Next" button can be pressed to continue now!
Hope this can save your time.