It does have mixing for elevons, I'm just not sure on how it should be set up.
Basically, you turn on the mixing.
Then you set up percentages for elevator, aileron, and differential.
And you're done. Assuming you know what these numbers should be, that is.

Here is the manual to the 7C (elevon info on page 45).
http://www.futaba-rc.com/manuals/7c-manual.pdf

Here is the manual for the plane I'm getting (in 3 languages, even).
http://www.ikarus-modellbau.de/Anleitungen/su27.pdf

The elevon section in the manual is fairly short and shows how to change the values. There is no info on what those values refer to.
The plane manual only goes into building the plane, not setting up what servo plugs into what channel and whatnot.
I'm gueesing it's going to be fairly easy via trial and error, but I'd like to know as much as possible before I fly it and find out the hard way.