Accessibility: Adding instruments in MuseScore 4
The concept of instrument selection is a little different in MuseScore 4. It's a three-stage process.
1. Tab once. You will get a choice of families of instruments, woodwind, brass, etc. You Choose the family by arrowing to the family you want. Note you haven't Selected anything, you have chosen the family.
Next, press tab again. This time you will get a choice of the instruments of the family group. So for woodwind, flute, oboe, etc. You again choose the members [not select them], by pressing the spacebar, as you did with the family group. This way, you can press the spacebar multiple times, so if you really want 34 flutes, you can have them.
You tab for the third time. This time you get a message that says Add instruments to Score. When you press the spacebar, all your chosen instruments will be added to your score. Your friends in MuseScore throughout are the tab key and the spacebar. They will help you with instruments, key-signatures, time-signatures, etc.
If you tab a fourth time in the instrument section, you can vary the order of the instruments shown on your score. And tabbing a fifth time, you will get a Cancel option. This is not what you want, so instead, move your right arrow key. You will get three choices, a cancel choice, a Next choice, and a Done choice. The chances are you will want the Next choice. So move your arrow there and press the enter key. I think you could also press the spacebar as usual, but I press the Enter key from habit.
Palettes are also easy. To access them, press shift+f9. The first option is a search box, so if you want to know how to add an expression mark, such as Piano, you could find it out here. But down arrowing will give you your clefs, key-signatures, etc. Once again, you use your arrows to choose what you want. The important thing to take from all this is that the word Selection is misleading. MuseScore is right to show that nothing is selected, simply because until you Choose what to do, nothing is.