Entering a chord with multiple note values (durations)

I've attached three small image files. Can anyone explain how to modify the score in current.png so that it looks and behaves like the score in goal.png? In the first chord, I want to change two quarter notes into whole notes. This is what I've tried so far:

(1) The note value buttons at the top of screen change all the notes in the chord, not just the ones I select.

(2) I can select the notes and change the "head type" in the selector, but upon exporting as MIDI, I do not see the changes (see midi.png). Perhaps the inspector just modifies the appearance of the note in the score?

What is the proper way to create a chord with multiple note values?