Jott Express doesn't say whether it's an OS X or XP app.
That's because it's neither; it's an Adobe AIR app.
Of course that means installing Adobe AIR on OS X*.
I've installed a few Adobe apps on OS X. Near as I can tell, Adobe is determined to destroy Apple. Adobe's installers routinely screw up OS X permissions, scatter files in illegal place, and generally act like drunken football hooligans.
Jott Express is also supposed to sync directly with an iPhone/iTouch. That's really piling the risk on.
I ain't installing AIR until I read some trusted source who's had a really good experience. I'd sooner invite Hell's Angels to dinner than Adobe products to my OS X box.
* In terms of cross-platform portable virtual environments there aren't a lot of great choices. Desktop Java is walking dead, Silverlight is from Microsoft, and ... well ... I think that's it.