First you need GB 10.0.1 because of a bug with apple sandboxing.
Secondly, This interface is pretty hard we will do a tuto maybe on hthis.
1. Select an intrument track
2. in menu view select "show smart controls" or something like this.
A panel should open at the bottom of the screen.
3. At the top left of this panel there is an "i" icon (like information) -> click on it
4. You should see at the right of the panel some details on the track appear.
5. on the bottom right of this detail sub-panel, you can see the current instrument, and inserts on the track, if you move your mouse over it you can add instruments.
A little helper :http://downloads.arturia.com/downloads/support/howto/pdf/arturia_audio_units_with_garage_band_10.0.pdfBTW, if you record a short screencast or screenshots on how to do it it can help others