You've got a couple of options here. You can either write in the midi parts for your virtual instrument using the midi editor (you can just double click to add a note) or you can actually set up your computer keyboard as a midi keyboard. To do this, add a new device in the Studio One/Options/External Devices/Add Device menu (Mac OS X: Preferences/External Devices/Add Device), choosing the QWERTY Keyboard device from the PreSonus device folder.