When using the Usage Based Optimization wizard (or the Aggregation Design Wizard), on the screen that asks you to enter counts. There is a button that says "Count" but after using it, you might still be confronted by the message ""Estimated Count or Partition Count must be specified for cube objects that require counting". It's not overly helpful, given you have just asked it to count them itself.
If you see this, look through the list of dimensions and find any that have red squiggles under them. Then open each and find the attributes that also have squiggles and update those counts. Make sure that all squiggles disappear.
You should then be able to continue to the next screen.
Hope this helps someone.