SSRS will automatically properly expand an "IN (@multi_valued_report_parameter)" if it's in an SSRS dataset and follows some basic rules. You may want to verify that you have a proper SSRS dataset.
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".