If you installed Leopard that is why you can't install Mt Lion. The system Must be running Snow Leopard 10.6.8 to upgrade to Lion or Mt Lion.
If you only erased the Macintosh HD partition then you left the Mt Lion Recovery HD partition in tact on that system. At startup hold down the Command+r keys to boot to the Recovery HD Partition. From there erase the Macintosh HD partition and then select Reinstall Mac OS X. It should then download Mt Lion and ask you for the Apple ID that was used to purchase Mt Lion.
The Reason Snow Leopard might not be installing is because of the Recovery HD partition. To get rid of that you must First Re-Partition the drive As One Partition. Then install Leopard then SL (Or Snow Leopard from the start) and then update it to 10.6.8. Once that is done you should be able to log on to the Mac App Store and Re-Download Mt Lion.
You use Disk Utility from a Boot source other then the built in hard drive. Like from a CD/DVD disc or USB thumb drive.
Boot the system from one of your OS X install DVDs and open Disk utility. Select the internal drive name, the TOP most entry, and click the Partition Tab. Select One Partition from the Partition Layout section and click Apply.