You lost me here with the keys down the side for the midi editor. That only happens in Music Studio, not Music Maker, as far as I can tell.
When you double-click on a midi object, the midi editor opens, fully. See the first image below. I cannot see how you could possibly only see one key, unless you managed to shrink the vertical view down. See the second image.
Below is the same editor but zoomed in vertically to the maximum, but it shows 2 keys. Click on the - as shown to zoom out.
Failing that, try another reset: File, Settings, Reset program settings to default.
In Music Studio, d-clicking on a midi object opens the midi editor below the arranger and you have to drag the window bar upwards to see more keys. If the above is not your problem, are you sure that you are not talking about Music Studio?