With help from Nonnus (thanks for great Support) i got iTouchMidi work with Garageband.

First you need a little freeware tool called "GarageRemote".
You can get it here: Garage Remote

It is a PrefPane that listens to MidiMessages and send, if triggered, a command to Garageband.
You can define MidiMessages for Back, Play/Pause, Rewind, Forward, Loop and Record.

You can use MidiPipe to find out which MidiMessage you have to insert.
You can get it here: MidiPipe

Example: I use the first 5 cells of the 8x8 Matrix to control Garageband.
Open MidiPipe, add "Midi In" and "AList" from the tools-list to the new pipe.
Select hex-view.
Tap on iTM-MidiLab the first cell. Two lines appear in MidiPipe: Note On and Note Off. The hex value is the value to insert in GarageRemote.

Do this with all controls and you're done!

Greetings Scherar