Try something like

with set [MIs] as

Filter([Marca_Modello].[Codice Merceologia].Members

,Left([Marca_Modello].[Codice Merceologia].[All Marca_Modello].CurrentMember.Name,2) = "MI")

SELECT

[MIs]

ON COLUMNS,

[Measures].[Importo Venduto] ON ROWS

FROM Vendite

WHERE [Anno].[All Anno].[2012]

Mack