Darren Gosbell was right on the nose in his comment
"I'm assuming from hints in your question that you are probably using SSRS 2005and SSAS 2005.
What sort of connection are you using?
If under "Type" it says "Microsoft SQL Server Analysis Services" then this will be your issue as I do not believe that this provider supports expressions in the query. If this is the case, change the provider to "OLE DB" and select the "Microsoft OLE DB Provider for Analysis Services 9.0" driver and this should fix your issue."
In a later response, he says the following
"Sorry, I missed the mention of 2008 - so the 10.0 provider is correct.
I just checked and I get the same error, but if you read the start of the error it says "Could not update a list of fields for the query..."
What I normally do with expressions in SSRS MDX queries is to run the query once with the expression usinghard coded parameters, this lets SSRS populate the field list.
Then I put in the quotes and string concatenation to build the expression, then you can ignore the warning about not being able to update the field list."
From yours truly...
Thanks a million Darren you saved my sanity.