Hi Eroxx,
First check that your audio interface is connected correctly and "seen by ableton. Go to preferences and select your audio interface as the audio input and output device.
click on input config and select every channel you see. It's a bit much and will put some strain on your computer, but it's rto make certain that ableton "catches everything that 's coming in and transmits on all audio output channels. Later you can mute them one by one to reduce this strain.
Next go to the link/midi tab in preferences and make certain that for input: matrixbrute (unknown mididevice everything is on
same for output : Matrixbrute (unknown mididevice)
Now go to the the main screen of ableton (where you see all your clips; select instruments/external instrument and drag external instrument to an empty clip channel.
In the the Midi from: section of the channel you should now see something happening when you press a key on the matrixbrute. To have the channel only recognise midi from the matrixbrute set Midi from: to Matrixbrute (unknown mididevice) Either "auto" or "in" can be selected. If you're not seeing any midi response in your newly create channel, you midi interface is not configured correcty or not working.
When you drag an external instrument to a to an empty channel as we did just now there will appear a device in the device strip in the bottom row. Check or change the settings to Midi to: Matrixbrute (unknown mididevice)
and audio from (select your audio interface) If you don't know what to check click on the audio from selection bar and play some sounds on your matrixbrute. The volume bars of the channel that receives the audio will light up. select this channel and set gain to the right level.
If all this works ok, click on the little floppy symbol in the top right corner of the device save the device as "Matrixbrute". You now have a working device that you can load every time you want to use the matrix brute as an external instrument.
In a similar way you can create devices for all your external instruments and set each one to a different midi channel this allows you to play them independently from ableton
Hope this helps..
Gert