I find that I have to have the note selected (yellow) and highlighted to change the note value.

As for the MusicXML import, there is a chance that the application that made the MusicXML is at fault. This is more common than you think. I suggest getting the SeeScore app by Dolphin Computing (there is a free version) and opening the MusicXML file with that app. If the MusicXML is incorrect in SeeScore, you know that the application that made the MusicXML is to fault, and is generating faulty MusicXML scores. If it appears correct in SeeScore, contact the customer service folks at Presonus/Notion, and let them know.

You could also send me an e-mail with the file and I could play with it...choirguy at a very common (googly) e-mail address...