When you embed custom code that contains references to Microsoft .NET Framework classes that are not in Math or Convert, you must provide an assembly reference to the report so that the report processor can resolve the names. For more information, see Add Code to a Report (SSRS).
To add an assembly reference to a report
In Design view, right-click the design surface outside the border of the report and click Report Properties.
In Add or remove assemblies, click Add and then click the ellipsis button to browse to the assembly.
In Add or remove classes, click Add and then type name of the class and provide an instance name to use within the report.
Specify a class and instance name only for instance-based members. Do not specify static members in the Classes list. For more information, see Custom Code and Assembly References in Expressions in Report Designer (SSRS).