Problem: when using datetime parameters on a report that has ssas as a datasource, the parameter will be presented as a string (long combobox with no hierarchy)
Solution: use the datasource filter tab to link your date field with the converted datetime parameter.
expression: =Fields!Date.Value
operator: =
value: =CSTR(Year(Parameters!Date.Value)) & "-" & CSTR(Month(Parameters!Date.Value)) & "-" & CSTR(Day(Parameters!Date.Value)) & " 00:00:00"
I found this the easiest solution, other solutions can be found here: http://www.mydatabasesupport.com/forums/ms-sqlserver/232393-parameters-ssrs-olap.html
