If you are trying to use the Custom Report Item sample in the updated SQL Server 2005 Samples, there was a mismatch in the early release of the documenation.
Specifically, there was an error in the configuration file entries. The documenation refers to a "PolygonsCRI" namespace while the sample project uses the "Microsoft.Samples.ReportingServices" namespace. You will be unable to load the custom report item in the designer unless you change the project settings or the configuration file.
The latest build of the documentation (as well as the online version) has the correct entry.