Mode Not supported stems for an resolution clash between the device connected and the tv. The most common issue when an tv is hooked up via hdmi to an device which does not support 480 i.
Are you using hdmi cord with your pc connection?
If your using an vga cable if so make sure the pc is set the optimum resolution. per the user manual (pc section)
I keep getting not supported mode coming up on monitor when i have laptop connected to my Samsung 4665 model tv. I get neither the laptop or the tv monitor to show my desktop? Anyone familiar with this problem?