How to do a lookup in RDL

The Lookup function returns the first matching value for the specified name from a dataset that contains name/value pairs.

Syntax
Lookup(source_expression, destination_expression, result_expression, dataset)  

Lookup based on Guid value

Nested tablix components can't use different datasets, so in order to cross reference values, from 2 different datasests, a Lookup function is needed

In order do a proper lookup based on Guid values, simply use the field values (without any other conversions).

Filtering a tablix rows based on a GUID value

Because the "Guid" selection type is not available in the designated filter dropdown, the recommended selection is "Text" 

Example running sample file

RDL sample file: GuidLookupSample.rdl