This is a known issue, we have already logged in our system for improvement. The fix will be provided in consecutive release of Telerik Reporting.
Meanwhile, you can copy the assembly locally in the folder of the Standalon Designer, and it will be loaded automatically. You need to specify the version, culture and publicKeyToken attributes of the assembly. For more details check the Extending Report Designer
help article, and the related resources at the bottom of the article.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.