You need to ensure that the Telerik assemblies you installed in GAC are correctly referenced in your project. Also, the version of both assemblies should be the same, you could use the following blog post to check if this is the case: http://blogs.telerik.com/aspnet-ajax/posts/12-03-28/getting-tired-of-seeing-trial-messages-in-your-asp-net-ajax-application.aspx
More information about the encountered issue can be found in the Skins Troubleshooting
help article: http://www.telerik.com/help/aspnet-ajax/introduction-troubleshooting-skins.html
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.