It seems that you have used the Report Parameter definition item instead of the processing counterpart. The definition report parameter value is actually the default value. The processing value is available in the report processing states but our suggestion is to bind the datasource parameter to the report parameter declaratively as shown in the ObjectDataSource Wizard and Using Parameters with the ObjectDataSource Component
Additionally if you want to access the report parameters processing value check out the Using Report Parameters programmatically.
the Telerik team