Hi! This is my first post here, although I own an N9 for some time now and I've been reading this forum before I even had a Maemo/MeeGo device...
Anyway, back to you. You can't install that app because it's for Nokia N900 (Maemo 5). For Nokia N9, I use this app Afinatron, and although it's in Portuguese, I'm pretty sure you'll be able to use it because the interface is very simple and intuitive.