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Yamaha S90 And 10.3. Need Direction!

Hello,
My family recently got an Yamaha S90. We are all novice piano players, but wanted to hook up our keyboard to our mac and explore that realm. We're running 10.3, and I get the keyboard to show up in Apple's Audio MIDI setup with no problems. Unfortunately, the software Yamaha ships with the keyboard is not mac os x compatible. I'm totally new to this MIDI thing, but I wanted to know what's a good software choice would be to be able to record what is played on the piano, add different instruments, and even do sheet music. We are not doing any production here. We want it to be simple and straightforward so that my 11 year old brother can use it. Does anyone have any suggestsions?

Any pointers regarding software, articles involving s90 and mac os x, or anything else are very much appreciated. Also, what types of things can I even do with a keyboard and a computer? I don't think I even have a clear scope in my head. Again, this is my first time ever doing anything MIDI, but I'm more than willing to learn.

Thanks,
Steven