Yamaha Keyboard Sound in Garageband

It is not possible, since GB doesn't send a midi signal back to your keyboard.

But if the keyboard makes it own sounds can't you just record from the line-out like you would any instrument?

You could just go from the line-out (or headphone-out or speaker-out) of the Yamaha into the Mac line-in. Be sure to choose "Built-in Input" in GB Preferences, Audio/Midi, Audio Input.
You may need an interface unit if you don't get a good enough signal, but I'd certainly try it.

Okay, this is confusing me and bringing up some questions.
He asked this same question in another thread and HT gave him basically the same answer I did. You're talking, I think, about MIDI signals, yes?
Since GB does not send MIDI out, does that mean one can't record the MIDI instruments in the MIDI unit to GB thru the MIDI output? I know that MIDI is not audio. But I'm still not 100% clear on how it all works.
But using the line-out to line-in bypasses the whole MIDI thing, right?

As others have said, you can always record the audio out of your Yamaha into Garageband onto a real audio track, just as you'd record a guitar or mic, but you can't use your Yamaha sounds as software instrument sounds, with Garageband "playing" your Yamaha via MIDI.

More full featured music applications, like Apple's Logic would enable you to play your Yamaha like a software instrument (play synth, synth sends MIDI to computer, computer records MIDI, on playback computer sends MIDI back to synth, synth plays audio), but I doubt the hassle of setting it up would be worth it. The virtual instruments in the computer will likely sound better than the sounds that come with that particular Yamaha.

It is not possible, since GB doesn't send a midi signal back to your keyboard.

Well, he said he wants to use his Yamaha sounds in GB, from that I take it he wants to play them as a software instrument!