Hi. Firstly, GarageBand is designed to NOT allow you to use any control surfaces (read knobs and faders) other than the iControl. There is a little 3rd party application called GarageRemote (google it) which will allow you to control the transport buttons from the Axiom. If this is of interest, there are full installation instructions on the M-Audio website. You will find it in one of the sticky's in the forum in the keyboard controller section.. look for GarageBand. I have never used it so cannot comment about its use. So unfortunately you cannot use the axioms knobs and faders...
The pads will work fine, I believe the factory default setting for pads 1-8 are C3, D3, etc meaning they correspond with the equivalent of middle C and that octave of keys. For making beats you will have to experiment with the position of the key range using the transpose button on the axiom.
Enigma is a editing program that allows you to set up and control templates or presets for your axiom and various DAW's. Unfortunately, this is of no use as GarageBand cannot utilize any preset functions.
You do not need any other software to use the Axiom with GB. Just plug and play!!
Cheers. G.