The app is called "Open Firmware password" even though MacBooks use EFI (I think that they are about the same). I have one for my MacBook. It is located in "Mac OS X Install Disk 1/Applications/Utilities/Open Firmware Password" just copy it to your hard drive and run it.
Well as Mernak said, you can set up a password that has the same effect as an Open Firmware password, but it isn't actually an open firmware password per se.