Completely erased my Leopard machine from another computer, how do I install an OS on it?
When you turn on the computer, put the disc in the drive with one hand, and hold down the option key with the other.  This forces the Mac to look for ANY bootable media.  If nothing comes up, the hard drive is blank (as you suspect), and the ROM drive can't read the disc you are putting in (or the ROM drive has failed).  You can try creating a Mountain Lion USB key.  Incidentally, what Mac are you putting this disc in?  A Mac Book Pro, an iMac?  What generation?  What CPU is in it?