is_good,
That's probably because the pre-requisite operating system or machine hardware is not present:
1. You can't install Lion (10.7) on top of a machine with 10.6.5 or less, unless there is no operating system present.
2. You can't install Mountain Lion (10.8) on a machine with 10.6.7 or less regardless.
3. You can't use the installer from the restore partition of a Mac that was prebundled with Lion or Mountain Lion on another Mac.
4. You can't install 10.6 gray discs on a Mac model of a different vintage or model than the gray disc shipped with.
5. You can't install 10.6 Update or Dropin discs on anything but a Mac that shipped new August 8 to August 29th, 2010.
6. You can't install 10.6 on a Mac with less than 1 GB of RAM (768MB is less).
7. You can't install 10.6 on a Mac with a G4 processor.
8. You can't install 10.7 on a Mac with a CoreDuo, or CoreSolo or G4 processor.
9. For Mac Minis, the Machine ID must be Mac Mini 3,1 or later to install 10.8 If it says 2,x 1,x, or PowerPC it can't install 10.8.
10. For 10.8, a minimum of 2 GB of RAM must be installed.
If none of these solve your problem, please start a new topic thread here.