This assemblies that are posted in this forum are a little bit outdated. I believe that OpenModalDialogs
is implemented in one of the later versions. We have just uploaded the latest internal build of our Silverlight 3 controls that is compatible against Silverlight 3 RTW. You can log into your account and download the files under the Downloads section. Download them and try them. They should fix the problems as the assemblies you were using are more than a month old.
Sorry for the inconvenience If you still have problems with the latest assemblies please let me know.
the Telerik team