Many times we want to associate a tablix with more than one dataset in reports but we are not able to do so in SSRS 2008 and previous versions. This limitation has been taken care of by some RDL expression language enhancements in SSRS 2008 R2. Lookup, LookupSet and MultiLookup are some really cool RDL expression language enhancements in SSRS 2008 R2. Let's see how the lookup works.
Suppose there are 2 datasets in report: DataSetA (ProductNameA,QuantitySold) and DataSetB (ProductNameB,TargetQuantityToBeSold) and we want to create a tablix report that shows ProductName,QuantitySold,TargetQuantityToBeSold and some Indicator (one more new feature in SSRS 2008).
Follow these steps in order to achieve this:

• Grab a tablix and drag the 2 columns from DataSetA on the tablix
• Add one more column to tablix and write an expression as: =LOOKUP(Fields!ProductNameA.Value,Fields!ProductNameB.Value,Fields!Target.Value,"DataSetB")

By doing this we have associated both the data sets with the tablix based on ProductName column (ProductNameA and ProductNameB). Click following link to know more: http://prologika.com/CS/blogs/blog/archive/2009/08/12/reporting-services-lookup-functions.aspx