The DirectX 10 Preview is very buggy, even default FSX scenery doesn't really work well with it, so it's usually best to keep it off. The only advantage of it, is that you can use the Bloom effect without an high performance penalty, but other than that, it's not very useful.
Lots of FSX addons, are not compatible with DX10 too, but all of them works in DX9 mode, which is the default.