How do I use lookup outside data region in SSRS 2008

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SSRS-2008

Hi,
I am new to SSRS and in one of the scenarios, I need to use Lookup function.
I have two datasets and I want to compare fields in these two datasets and get the ouput.
Let's say I have datasets ds1 and ds2. I am trying to match tag name from ds1 with itename from ds2 and get the value from ds2.
Below is my lookup function.

=Lookup(Fields!Tag_Name.Value, Fields!Itemname.Value, Fields!Value.Value, "ds2")

I get below error.

System.Web.Services.Protocols.SoapException: The Value expression for the text box ‘txtOpenTemp’ refers directly to the field ‘Tag_Name’ without specifying a dataset aggregate.  When the report contains multiple datasets, field references outside of a data region must be contained within aggregate functions which specify a dataset scope. 

My textbox is outside the data region and I guess because of that I am facing this issue.

What I have tried:

I could try for aggregate functions like

First(Fields!Tag_Name.Value)

but this will compare only first value always.