The RadFileExplorer will accept only application relative paths for the View, Upload, and Delete paths properties (e.g. paths that start with ~/folder/ or /folder/). If you want to use it to access files outside of the web application, you will need to write a custom content provider. Here is an example that shows how to use it with an SQL database: http://demos.telerik.com/aspnet-ajax/fileexplorer/examples/dbfilebrowsercontentprovider/defaultcs.aspx
. Here is a page from the control help, which describes the process of creating a custom provider: http://www.telerik.com/help/aspnet-ajax/radfileexplorer-custom-filebrowsercontentprovider.html
For your RadEditor question - if I understand correctly, you want to use the Image Manager to show files from SharePoint libraries. This is already possible with the RadEditor for MOSS product, which you can get from the downloads page for the RadControls for ASP.NET AJAX. The MOSS editor has a custom content provider that connects with SharePoint libraries instead of physical files in the web application. This way you can browse and upload files directly in the libraries.
the Telerik team