This Help Center is about using MuseScore.com. For help using the MuseScore software, you'll want to search the MuseScore.org Handbook. Also on MuseScore.org are the forums, where experienced MuseScore users will often answer your question within just minutes.
If the whole MuseScore/MuseScore.org/MuseScore.com thing is confusing you, see What is MuseScore?