After you hit the Buss assign button hit the funtion key under OUT. On the top right there is a list of all the different busses and what they are assigned to. You need to select the Buss and use the scroll wheel to select which buss you want to assign.
I usually assign the 7/8 buss to MASTER for mixdowns. Select the 7/8 buss on your recorder and you will have the main outs.
I am at work and this was from memory so let me know if you need more help. I'll check it out when I get home.