Hi Fabian,
We have just published a blog post, which explains how to use the file explorer in SharePoint. The post includes a link to the SPContentProvider custom content provider you can use to connect the control to the SharePoint libraries in your site. Here is the post link:
http://blogs.telerik.com/blogs/09-06-17/using_the_radfileexplorer_for_asp_net_ajax_in_a_moss_web_application.aspx
Regards,
Lini
the Telerik team