The virtual keyboard in Mai Tai is there to audition the virtual instrument and cannot be used to record MIDI directly into an instrument track. You will want to either go to View>Editior and manually draw the MIDI notes into the program, or use the Qwerty keyboard to record MIDI, if you do not have a MIDI keyboard. The Qwerty keyboard option for MIDI entry can be found by going into Studio One>Options (Preferences on a Mac)>External Devices. When you add a device, just select the Qwerty keyboard from the "PreSonus" folder. Make sure you set this as your default device, by checking the appropriate box when setting up the device. For the best experience, you may want to consider purchasing a MIDI keyboard.