1) How do I get SOUND from the keyboards into the Mac - this is AUDIO I/O
2) How do I CONTROL the Mac from the keyboards (and the keyboards from the Mac)? This is MIDI I/O
1) To get sound in and out of the iBook you can use the Audio In and headphone jacks, but you will get better quality by using an external USB or Firewire Analog/Digital audio interface. The Griffin iMic is the entry level, Edirol, M-Audio, Steinberg and others make more capable units. Price $50 - $2000 depending on number of channels and features
2) To get MIDI control signals in and out of the Mac, you need a MIDI Interface, which is typically USB. MIDI doesn't carry any sound information, it carries data on note on/off timing and control values. Think of the paper roll in a player piano - MIDI carries the instructions to "play" the instrument.
Sometimes, keyboards have USB ports for interfacing directly with a computer (the keyboard essentially ascts as a MIDI interface) however I dont think your 2 do. Again, Edirol and M-Audio have MIDI interface products, the Edirol UM1X is the cheapest http://www.edirol.com/products/info/um1sx.html