To check if a product is compatible with your simulator (or hardware), check the product page and scroll down to the System requirements tab found next to the product description. If your simulator is not listed in the requirements, you should assume the product will not be compatible with your simulator.
FSX and FSX:Steam Edition are not the same versions, and as such, products only supporting one version may not support the other.
Prepar3D products are always only compatible with one specific version. For example, if P3D V4 is listed, the product will not work on P3D V3 and vice versa. If you see Prepar3D without a version number, assume it is the original release of Prepar3D as It did not have a version number.