In Studio One when you start a new song, select where it says "Interfaces" scroll down and select AudioBox 1818VSL.
Then all of your Inputs and Outputs will show up.
Otherwise if you start a blank song you will only get Inputs 1 and 2 as that is what is setup by default in Song - Audio I/O Setup.
Read the AudioBox 1818VSL User Guide to learn how to add additional tracks on your own if you don't want to use the Template within Studio One. Page 57 (Step #9) to be exact. AudioBox 1818VSL User Manual. We suggest starting at page 47 though to get the full scoop on how to use your interface with Studio One correctly.