MacBook Pro won't start up

That folder with the question mark or slash icon means that the MacBook can't find the boot directory. That can either mean it can't find the hard drive or the Operating System data on the hard drive is somehow corrupted.
Put your install DVD into the optical drive and reboot. As soon as you hear the boot chime, hold down the "c" key on your keyboard (or the Option key until the Install Disk shows up). That will force your MacBook to boot from the install DVD in the optical drive.

When it does start up, you'll see a panel asking you to choose your language. Just press theReturn key on your keyboard once. It will then present you with an Installation window. Completely ignore this window and click on Utilities in the top menu and scroll down to DiskUtility and click it. When it comes up is your Hard Drive in the list on the left?
If it is then click on the Mac OS partition of your hard drive in the left hand list. Then select theFirst Aid Tab and run Repair Disk. The Repair Disk button won't be available until you've clicked on the Mac OS partition on your hard drive. If that repairs any problems run it again until the green OK appears and then run Repair Permissions.
If your hard drive isn’t recognized in Disk Utility then your hard drive is probably dead.

sig is asleep at the wheel and gave you instructions for 10.6 when you have 10.7.4 where there are no disks. 🙂

Go through the fix list here, need to do #4-#9 likely

If this is beyond your capabiilty, then take the machine in for service

Step by Step to fix your Mac