We did away with the RadControls method of managing the editor as Telerik has deprecated that functionality. The editor is now added to pages as a control, as in the code you posted. To add other folders, just add them to the paths specified there. For example, to add the StringResources folder:
Code:
<telerik:radeditor id="radDescription" runat="server" editable="true">
<ImageManager UploadPaths="~/StringResources, ~/Images" ViewPaths="~/StringResources, ~/Images" DeletePaths="~/StringResources, ~/Images" />
<DocumentManager UploadPaths="~/Images" ViewPaths="~/Images" DeletePaths="~/Images" />
<FlashManager UploadPaths="~/Images" ViewPaths="~/Images" DeletePaths="~/Images" />
<MediaManager UploadPaths="~/Images" ViewPaths="~/Images" DeletePaths="~/Images" />
<SilverlightManager UploadPaths="~/Images" ViewPaths="~/Images" DeletePaths="~/Images" />
<TemplateManager UploadPaths="~/Images" ViewPaths="~/Images" DeletePaths="~/Images" />
Note that you'll need to give the .NET user account (usually ASPNET or NETWORK SERVICES) read/write/modify access to whatever folder(s) you specify as well.