First you have to create as many multi-instruments as you need in the environment. Of course you can do the whole process one by one too.
First you create a multi-instrument in the environment window. You can choose to give it an appropriate name.
Then you route this multi-instrument to your first instrument channel by dragging from the small white arrow to the instrument channel.
What I forget to mention in my first post is that in this channel you have to choose a VE instrument, otherwise nothing is loaded of course. But this is pretty obvious for everyone I guess.
Then you go on just the way I described.
I have posted a small video on this by Apple on my myspace page in the video section. Maybe this will be helpful too. I don't remember where I found it some time ago.
jef
www.myspace.com/aeronautsounds