While running the Agg Design Wizard I kept receiving the following error: "Errors in the metadata manager". It was driving me nuts. Finally I ended up doing a search for the error string and came across this fix:
1) Remove the custom assembly from your project in Business Intelligence Development Studio.
2) Deploy your project.
3) Design the aggregations for your partitions with the Aggregation Design Wizard.
4) Add your custom assembly back into your project in Business Intelligence Development Studio.
5) Deploy your project.
Thanks to this link for the solution: http://www.mrachek.com/PermaLink,guid,af10cb31-0a3c-457b-89bb-06c5f26bdf8a.aspx