Hmm...it normally should be pretty straightforward, so unfortunately, don't have a lot of great ideas left.
If it helps at all, a screenshot from my G46VW running Win8 with external monitor connected through HDMI is below.
My system is running the 210 bios with the original nVidia vga driver 9.18.13.546 (which isn't posted on the ASUS G46VW download site although the posted version should work okay too?) that came with the system and the Intel vga driver 9.17.10.2828.
Make sure the HDMI cable is firmly plugged into the notebook of course, but I couldn't get the external HDMI output to appear in the bios, so we may need to make sure everything is normal in Win8 and there aren't any problems with the vga driver versions.
Once Win8 booted, the external display was automatically duplicated for my unit and I used the Win8 'devices' charm to extend the display although other ways should work too.
Well, hopefully others have better ideas if it's still not working.