you hear a few tones/beeps during startup. After they end and the screen goes black would be the point at which you do this.
However, because your MB has both a DVI out and a HDMI out, your probably going to need some linux knowledge to get it to display out on the HDMI. I'm not sure how much help you will find because of the non-standard MB used.
If you have one handy, you could try a DVI to HDMI just to see if that works when using the method above to press #1. Alternatively, if your TV has a DVI or VGA port you could try an DVI to VGA adaptor or simply a DVI cable.
There might be another option, see if you can disable the DVI port in the BIOS? (doubtful, but you never know).
If you are dead set on using the HDMI port as is, I believe your going to have to edit the xorg.conf file, but someone a little more knowledgable with Linux and LMCE should probably jump in because at this point its going above my knowledge point.