How to Use Custom Assemblies in SQL Server 2005 Reporting Services

Once you have created a custom assembly you will need to copy the .Dll to the following Folders:

[Drive]:\Program Files\Microsoft Visual Studio 8\Common7\IDE\PrivateAssemblies
[Drive]:\Program Files\Microsoft SQL Server\MSSQL.3\Reporting Services\ReportServer\bin

VS 2010 x64 machine:
[Drive]:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\PrivateAssemblies

Reference:
How to use custom assemblies or embedded code in Reporting Services