If you are not using the retail Snow Leopard DVD, then it won't work. If you are using the retail DVD, then before you can attempt to install it you must use it to erase the drive and repartition it.
Clean Install of Snow Leopard
1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came
with your computer. Insert the disc into the optical drive and restart the computer.
After the chime press and hold down the "C" key. Release the key when you see
a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue
button. When the menu bar appears select Disk Utility from the Utilities menu.
After DU loads select the hard drive entry from the left side list (mfgr.'s ID and drive
size.) Click on the Partition tab in the DU main window. Set the number of
partitions to one (1) from the Partitions drop down menu, click on Options button
and select GUID, click on OK, then set the format type to MacOS Extended
(Journaled, if supported), then click on the Apply button.
3. When the formatting has completed quit DU and return to the installer. Proceed
with the OS X installation and follow the directions included with the installer.
4. When the installation has completed your computer will Restart into the Setup
Assistant. After you finish Setup Assistant will complete the installation after which
you will be running a fresh install of OS X. You can now begin the update process
by opening Software Update and installing all recommended updates to bring your
installation current.
5. If you are planning to sell or give your computer away, then do the following:
After you reformat your hard drive and reinstall OS X, the computer restarts to a
Welcome screen and asks you to choose a country or region. If you want to leave
the Mac in an out-of-box state, don't continue with the setup of your system. Instead,
press Command-Q to shut down the Mac. When the new owner turns on the Mac,
the Setup Assistant will guide them through the setup process.
Five ways to eject a stuck CD or DVD from the optical drive
Ejecting the stuck disc can usually be done in one of the following ways:
1. Restart the computer and after the chime press and hold down the
left mouse button until the disc ejects.
2. Press the Eject button on your keyboard.
3. Click on the Eject button in the menubar.
4. Press COMMAND-E.
5. If none of the above work try this: Open the Terminal application in
your Utilities folder. At the prompt enter or paste the following:
/usr/bin/drutil eject
If this fails then try this:
Boot the computer into Single-user Mode. At the prompt enter the same command as used above. To restart the computer enter "reboot" at the prompt without quotes.