I made a couple of changes in the object data source sample on www.gotreportviewer.com to illustrate the use of nested objects in report fields
I have added a ProductSales sub-class to the Product class. ProductSales has two fields: Quantity and Price.
In the report - you can reference them via:
Fields!Sales.Value.Quantity and Fields!Sales.Value.Price
You can similarly use multiple levels of object nesting, for example:
See attachment for details.