First of all, can you tell us whether your request is related to Sitefinity or just for a regular ASP.NET application where you are using the Rad Controls, because there is a difference between the two applications.
Here is a quick example:
1) Lets say that you create a user control Breadcrumb.ascx and create a resource class for it Breadcrumb.resx. You need to add the resx file in ~/App_Data/Sitefinity/GlobalResources folder.
2) Now the Sitefinity resource loader will work as the built in ASP.NET Global Resource provider and will read the resource classes.
All the best,
the Telerik team
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