I’m using Note Performer 3 with Dorico 4 and have a Percussion part set up as a kit utilizing suspended cymbal, sleigh bells and bass drum. I am trying to add flexatone, but when I add it to the kit it doesn’t show up on the score on the same line I added it to when I created it, and it sounds like a bongo, not a flexatone. What am I doing wrong?

You should check in NotePerformer’s documentation whether or not it includes a flexatone instrument.

A quick check of the instrument list on NotePerformer’s home page shows that the Flexatone is not included.