What type of popup do you use: AjaxToolkit modal popup extender, jQuery dialog, RadWindow or other dialog?
Does the dropdown popup of the FontName tool appears at all? If not this means that its z-index is smaller than this of the dialog.
Using the CSS class below you can fix the problem:
You need to add the style declaration to the page which hosts the RadEditor control.
If this does not solve the issue or the reported problem is different, could you please send me a fully working project which shows the problem. I will examine it and provide a solution as soon as possible. You can send the project to email@example.com or open a support ticket.
the Telerik team