The Music library does not have MuseScore downloaded on the computers, so you can't take advantage of our MIDI keyboards using this program, but if you export something you have created in Finale as a music xml file, you can import that into MuseScore and vice versa. Some cosmetic things may shift between programs, but the main elements of the piece will remain. This can be tricky to work on because the keyboard shortcuts are often different, but if you're in a pinch, sometimes this can be a real help.
Double clicking anything will allow you to adjust the element, normally its size and placement on the page. If you want to delete it, click out of the element and then click once to highlight it and then press the delete key.
MuseScore is an open source software and because many people work on making it better all the time, the community supporting the software is vast. It is fairly easy to find the answers to your MuseScore questions through a google search, or by scrolling through the MuseScore Handbook.