you can enter notes manually using the mouse in the Piano Roll View (PRV), but not using your computer keyboard. just open the Piano Roll View in X3 for that MIDI track .I think double clicking on the track defaults to opening the PRV, but I'm not 100% positive on this - if that doesn't work, check VIEW>PRV or something similar.
there is a 3rd party software you could buy to input notes using your computer keyboard, but there's no real control of velocity and it's not cheap ($50, IIRC), so IMO it would be money better spent to go buy a cheap keyboard for that amount of money with velocity.