I'm not sure if there is complete Z550 support in XS++. I have used it with the K750, W800 and W810 several times without any problems. And I am about to try with a W700 and K700 within this week. Also did you flash it with the correct break-in firmware before running FSX?
Bump: I've got a partially working alpha of the Z550 break-in firmware if you want it. Just drop me a PM.