Missing Instruments?

MIDI doesn't define "Instruments", it defines sounds for playback. Actually, MIDI doesn't go that far, an additional standard called "General MIDI" does that. It defines a set of sounds that all GM-compatible synthesizers must support. Many of those sounds aren't instruments in themselves but particular sounds made by instruments. For example, "Trumpet" and "Muted Trumpet" are not two different instruments, just two different sounds made by the same instrument. Same with a variety of different sounds defined for guitars, basses, stringed instruments, percussion instruments, and most especially synthesizers. Also, there are *very* many more instruments than General MIDI defines a sound for.

So it is important to keep these things straight in understanding how MuseScore works. The Instrument list is where you select the basic instrument to be used in terms of establishing the initial clef, transposition, and playback sound, but the if you wish an instrument to make a *different* sound from its default, you choose that alternate sound in the Mixer.

Anyhow, all that said, even though Pad Synthesizer is *not* really a specific instrument, as a convenience, an entry *is* provided in the isntrument list for it. It's basically just a piano in terms of the main notation settings, but with the initial sound already set for you. To find this, switch to "All instruments" in the drop down at the top of the dialog, or just start typing a few characters of the name into the Search box at the bottom.

EDIT: I just tried it, and this "Pad Synthesizer" instrument actually does other interesting things. It defines nine separate "channels" for the isntrument to allow easy switching between eight different General MIDI sounds (88-95) via the same mechanism (Staff Text) that you can use to switch between trumpet and muted trumpet.

For more info, see the Handbook under "Sound and playback".

You should be aware that the GM patch definitions do not define a particular sound, but are a guideline to the kind of sound a musician should expect when choosing that patch number. This is particularly applicable to the synth sounds (#81-#104).....

http://www.midi.org/techspecs/gm1sound.php

If you are wishing to have a particular sound in your compositions, then you need to source and install a soundfont containing that sound into MuseScore.

Be aware, however, that playback on a MuseScore installation without that particular soundfont will not sound the same.

The default FluidR3Mono soundfont conforms to the GM Level 2 standard, and contains patches following the MMA guidelines, together with variation banks, again as defined in GM Level 2.

Individual patch control is accessible from the Mixer window.

Incidentally there are no MIDI standard instruments. There are, however, standard instruments defined in General MIDI Levels 1 & 2.

Hope This Helps.