you should go to mackie.com and go to their user forum or maybe get support form mackie. Questions like the one you are asking wont get answered because its something you should be able to find out by reading the manual or from contacting the companies customer support services.

you need to go into the mixer and correctly assign each track to the correct input on the mackie.

You've installed the drivers from mackie right? You should see the mackie device listed somewhere under "sounds and audio devices" in the control panel of your pc. (I'm assuming you're using windows). Look for something about "vst-buses" or "routing" in the html help within cubase. You'll have some basic setting up to do. Once you're done, whenever you click on the "input field" on any channel strip you'll get a dropdown list of all the available inputs (1-16). Just choose different inputs for different tracks. Here's a tip, you have to power up your pc before you turn the mackie on. For some reason windows will only recognize the Mackie if it's turned on after windows is running.