How to save in 5.1

I am working with 5.1 FLAC files. I am trying to mix tracks together. However, when I try to save them, I get an error message that it is going to mix them to mono. How do I save back to 5.1 FLAC?

[u]Use Advanced Mixing Options[/u].

The output channels aren’t identified as their surround locations (and I don’t know either) but if you’ve imported a 5.1 file they come out right.