Boot into the disc that came with your MacBook by inserting it in the drive and holding down "D" when you hear the startup chime. When it gets to the welcome screen for the installation (after you select the language, of course), go to the menu and check the option to reset the password. A window will come up allowing you to select which account you want to change the password for. Once you've done that, cancel the installation and the Macbook will prompt you to restart. Let it restart from the hard drive and try logging in with the new password.
Good luck.