The audio setup in Cubase works that for each in and out on your Interface you can either add them in Stereo busses two at a time, or mono busses one at a time, or both. You can then type in a custom name for your setup and save it as a preset. It's different than a lot of other DAW's I've seen, but let's say you have an 8x8 audio interface and you are only seeing stereo bus 1 input 1 and 2, you just need to hit the Add stereo buss 3 more times and add stereo pairs 3/4, 5/6, 7/8 and/or add mono bus 1 - input 1, mono bus 2 - input 2, etc. just keep adding mono buses to add the devices. Same thing on the output tab, you add busses for the number of outs, either in stereo or mono, or both. it's tedious at first, but save your preset, so you can recall it at any time.
And for the "zones" as Cubase calls them, just do as Sir William said, the 3 icons in the upper right hand corner. One confusing thing that I found is the mix zone doesn't detach, you just hit F3 to bring up a separate mixer pane. So you can actually have a mixer pane and the mixer zone up at the same time, which is actually kind of cool if you run two monitors and could come in handy.