Well there are 2 ways of doing it actually. One is embedding it inside the report itself, but you are limited to using only VB.NET. The other one is creating a custom code assembly, either in VB.NET or C#, and referencing it from the report. If you ask me, I prefer the custom code assembly so I can write it in C#.
The following links go through the step by step of adding custom code:
- SQL Server Reporting Services Embedding .NET Code for Report Formatting and Error Handling
- SQL Server Reporting Services Custom Code Assemblies
For more information on MSDN, click here.