Hi,
I know that this reply is coming a good two-and-a-half weeks later. But you could try this:
Set the aggregation for the measure to None instead of Sum. Then you need to setup some business intelligence by going to the Cube menu, select Add Business Intelligence. This would open up the appropriate wizard. Select the Create a custom rollup formula option in this wizard, next select the Parent-child dimension and then the attribute that defines the parent-child relationship. Select an appropriate source column (such as the Name attribute of the dimension) and you're done. Process the cube and check the results.
Note that you need to have a time dimension in order to get proper results, since the measure has become a non-additive one. Also, if the BIDS browser dosn't seem to show proper results try retrieving the data from Excel 2007. If you need I could give you a little sample of what I came up with.
Best Regards

Gogula G. Aryalingam (MVP - SQL Server)