How to open MIDI sequencer/Channel/Program in MuseScore?

Can you explain what you mean? MuseScore is not a MIDI sequencer, but if you have a standard MIDI file (".mid") that was created by some other MIDI sequencer program, you can open that MIDI file just like you'd open any other file (File / Open), and MuseScore will do its best to turn it into something resembling readable notation. Be aware, though, that MIDI is not a notation format and lacks basic information that would be needed to do a really good job of this, and you'll often need to fix things up manually.