There's two things you can do:
1- Drag the edge of the mix tab up until you see it (expand the faders to the top).
If that doesn't work:
2- Open the options on the side of the mixer. There should be and option like "see I/O" or "View I/O" or something like that (I use SO4 in spanish. Sorry).
In case that doesn't work you can post another question and say that you already tried those two options.
I hope I was of help.