Ok, for starters make sure you are in Nero Burning ROM and *not* the "Smart Start" aka "Nero Express" menu by going Start>Programs>Nero>Nero (version #)>Nero Burning Rom.
Once started, the "new Compilation" menu will pop-up. Cancel out of that dialog. Then select file>open and point to the path where your *.iso is. Tick the Finalize CD/DVD box and click Burn button. That's it.