The Addon Manager doesn't have anything to do with this (it doesn't have anything to do with almost everything right now...) since the 3d editor runs entirely external to it, and if there were any problems with your install, nothing would work.
I can only confirm it works fine here, the only possible explanation, if you haven't changed the default Slew key, is that another addon you have installed has subscribed to catch the Slew key so, it get the event but it doesn't pass it away, as it should.
The correct behavior for an add-on that intercepts a default key, is to give it back to FSX when it's done with it, to allow use by FSX itself or other addons that might need it too, but if it doesn't do that, the other addons won't get notified of the key, so they won't work anymore.