Problems with the miniport display thunderbolt-HDMI

I have an iMac 3.4 Ghz Intel core i7, the software is Mac OS x 10.6.8 and I have a new TV HD LG 32" (LG 32LS5600- ZC) with 3 HDMI plugs. When I put the mini Display port in the thunderbolt plug behind the MAC and in the HDMI port of the LG tv nothing happens... why? I tried to switch on before the TV and after the Mac... the opposite... and to check in Preferenze del sistema to see if the voice rileva monitor worked... nothing. The HDMI cable works because I am able to see the satellite TV perfectly. The mini port display Kanaan is new, bought on Amazon few weeks ago... The Mac doesn't see the TV monitor which says: nessun segnale whatever I do!

May you help me?

Thanks