What you have to have in mind about the WHERE clause in MDX is that in it you are supplying the context for the query (I think originally it was CONTEXT rather than WHERE). So it's not like the SQL WHERE something IS something.

With that in mind have a look into the FILTER function and you should be able to achieve what you need to.

http://technet.microsoft.com/en-us/library/ms146037.aspx

I could write the query for you but I think it stays with you better if you learn it yourself 😀

Best of luck.