Ok we're assuming your x2vga power is on and dipswitch 4 is set correctly. Is your box modded? Go into your modded dashboard and then to the MS Dashboard from there (I'm using EvoX which has a link to the original dashboard) and then go into your video settings and it's all there.
Make sure the x2vga is connected and you're viewing successfully with a monitor through the VGA connector and not through composite - if composite is on the VGA circuitry shuts down (toggle dipswitch 4).
If you still can't see it, it sounds like your modchip maybe doesn't support changing to NTSC mode? Try going through your modchip/bios settings to change the video to NTSC (though mine seems to work fine set to PAL - Xenium chip).
If all of that fails, can you supply more information - exactly what are the dipswitches set to, what cables have you plugged in where, what happens when you boot, mod chip and dashboard details, where do you go in the menus to try and enable HD etc.... we need more information mate
