In order to use computer keyboard note input in Igor Engraver one has to use a synth together with Igor Engraver and set the correct input port for the synth in Igor Engraver.
1. Start the synth, e.g. a software synth like Qsynth.
2. Start Igor Engraver.
3. In Igor Engraver, open Edit -> MIDI -> Synth settings.
4. In the Synth settings window, open the drop-down menu "Synth" and choose e.g. "Synth input port (Qsynth1:0)".
5. Click the buttons "Prefer synth for instrument" and then "OK".
If one wants to, it is possible to close the software synth and continue using computer keyboard note input in Igor Engraver, but then of course without sound.