MacBook won't start up.

How would we know? We don't know what you did, and neither do you.

Can you tell me the actual filename and path? Then I may be able to tell you what the entries are supposed to be. Unless you happen to know in which case simply change them back, if you can.

It's possible if you have another Mac you can connect via Firewire cable or an external hard drive from which you can startup the computer.

Here's mine but it's from a Mountain Lion system:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

<plist version="1.0">

<dict>

<key>Kernel Flags</key>

<string></string>

</dict>

</plist>

My guess is you just need to delete both of your additions to the file:

<string>mbasd=1</string>

<key>Kernal</key>

<string>mach_kernel</string>

However, you need to get into the hard drive to do this. This would require either another computer with Firewire or an external drive with a bootable system on it.

As an alternative you can boot from your OS X Installer DVD. After the installer has loaded you can run Terminal from the Utilities menu in the menubar. Then you can try navigating to the location where the file is located and making the needed change.

You will need to do a CD "/Volumes/Macintosh HD/Library/Preferences/SystemConfiguration" before you do anything. I'm assuming your disk is named Macintosh HD. If not, then substitute the name you use.

Only if it will boot despite the change you made. Boot the computer using Single-user Mode.

If nothing I have mentioned has helped, then I can only suggest you reinstall OS X. You are running Leopard, apparently, so you can try the following to avoid erasing the drive:

How to Perform an Archive and Install

An Archive and Install will NOT erase your hard drive, but you must have sufficient free space for a second OS X installation which could be from 3-9 GBs depending upon the version of OS X and selected installation options. The free space requirement is over and above normal free space requirements which should be at least 6-10 GBs. Read all the linked references carefully before proceeding.

1. Be sure to use Disk Utility first to repair the disk before performing the Archive and Install.

Repairing the Hard Drive and Permissions

Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Installer menu (Utilities menu for Tiger, Leopard or Snow Leopard.) After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer. Now restart normally.

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

2. Do not proceed with an Archive and Install if DU reports errors it cannot fix. In that case use Disk Warrior and/or TechTool Pro to repair the hard drive. If neither can repair the drive, then you will have to erase the drive and reinstall from scratch.

3. Boot from your OS X Installer disc. After the installer loads select your language and click on the Continue button. When you reach the screen to select a destination drive click once on the destination drive then click on the Option button. Select the Archive and Install option. Click on the OK button and continue with the OS X Installation.

4. Upon completion of the Archive and Install you will have a Previous System Folder in the root directory. You should retain the PSF until you are sure you do not need to manually transfer any items from the PSF to your newly installed system.

5. After moving any items you want to keep from the PSF you should delete it. You can back it up if you prefer, but you must delete it from the hard drive.

6. You can now download a Combo Updater directly from Apple's download site to update your new system to the desired version as well as install any security or other updates. You can also do this using Software Update.

I'm not surprised since single-user mode uses the same .plist file you edited. You will have to try using the Terminal after booting from the installer DVD.