When you add an Crosstab (don't use the main dataset but choose: 'new dataset' and use new one with new query), and then go to the
* Submenu Row Groups
* Submenu (chosen rowgroups) 'myRowGroup' a.e.
And then go to the properties
* Expression (beneath 'Bucket')
* Open Expression editor
You only see the 'fields' from the main dataset. You can actually use the fields from the dataset you used for the Crosstab, but they are red underlined (as if they are wrong) and not visible in the fields section.