Keyboards usually have an output for headphones, or even left and right stereo output for amplifiers depending on what kind of keyboard you've got. So you simply need to find a double-sided cord to plug into your keyboard's headphone jack, and then input into your computer through the microphone jack. Then you can record with your keyboard. A MIDI cable would basically give you control of recording MIDI notes into a program that you could then edit or whatever, which is really not what you're trying to do.... You're trying to record some real audio with vocals overtop. So seeketh that double-sided input cord, sir.
As for a program, I recall there being this nice small program called Acoustica something that is very small and would be perfect for you. It's not really necessary to get a big laggy program like Audition 2.0 or whatnot to handle something small like vocals, dialogue, and keyboards. So Google Acoustica and find the link for Acoustica MP3 Audio Mixer. It's what I started out with, what, 8 years ago.