It is the Edition of MS AS that you're running, most likely Standard Edition (do a search for 'Features Supported by the Editions' in BOL to see what each edition supports).  So the bottom line is that without changing editions, calculated cells is not going to be an option.

After looking at BOL again, you may want to look at Custom members within the dimension.  BOL has an example ->

IIf(Employees.CurrentMember.Parent.Name = "SalesPersons", Sales * 0.10,

    RollupChildren(Employees.CurrentMember,

    Employees.CurrentMember.Properties("UNARY_OPERATOR"))

that looks reasonably similar to what you're trying to achieve, you would of course need to do a check for both the measure name being used and the member being used.